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03 IP Subnetting and Variable Length Subnet Masks - PDF

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03 IP Subnetting and Variable Length Subnet Masks - PDF Powered By Docstoc
					  CCNA
     640-801




IP Subnetting and Variable Length
     Subnet Masks (VLSM)




               Revision no.: PPT/2K605/03
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                                                                                                                                            Subnetting Basics
                                                                                                                                                  There are loads of reasons in favor of subnetting. Some of
                                                                                                                                                  the benefits include:
                                                                                                                                                        Reduced network traffic:- We all appreciate less traffic of any
                                                                                                                                                        kind. Networks are no different. Without trusty routers, packet
                                                                                                                                                        traffic could grind the entire network down to a near standstill.
                                                                                                                                                        Optimized network performance:- This is a result of reduced
                                                                                                                                                        network traffic.
                                                                                                                                                        Simplified management:- It’s easier to identify and isolate
                                                                                                                                                        network problems in a group of smaller connected networks
                                                                                                                                                        than within one gigantic network.
                                                                                                                                                        Facilitated spanning of large geographical distances:- Because
                                                                                                                                                        WAN links are considerably slower and more expensive than
                                                                                                                                                        LAN links, a single large network that spans long distances can
                                                                                                                                                        create problems in every arena listed above.
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                                                                                                                                            How to Create Subnets
                                                                                                                                                  To create subnetworks, you take bits from the host portion
                                                                                                                                                  of the IP address and reserve them to define the subnet
                                                                                                                                                  address.
                                                                                                                                                  To determine your current requirements as well as plan for
                                                                                                                                                  future conditions. Follow these steps:

                                                                                                                                                        Determine the number of required network IDs:
                                                                                                                                                               One for each subnet
                                                                                                                                                               One for each wide area network connection
                                                                                                                                                        Determine the number of required host IDs per subnet:
                                                                                                                                                               One for each TCP/IP host
                                                                                                                                                               One for each router interface
                                                                                                                                                        Based on the above requirement, create the following:
                                                                                                                                                               One subnet mask for your entire network
                                                                                                                                                               A unique subnet ID for each physical segment
                                                                                                                                                               A range of host IDs for each subnet
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                                                                                                                                            Subnet Masks
                                                                                                                                                 A subnet mask is a 32-bit value that allows the recipient of
                                                                                                                                                 IP packets to distinguish the network ID portion of the IP
                                                                                                                                                 address from the host ID portion of the IP address.
                                                                                                                                                 Classdefault subnet masks for Classes A, B, and Subnet Mask
                                                                                                                                                 The     Format                           Default C. These
                                                                                                                                                 default masks cannot change.
                                                                                                                                                 A       network.node.node.node           255.0.0.0
                                                                                                                                                 Default Subnet Mask
                                                                                                                                                 B       network.network.node.node        255.255.0.0
                                                                                                                                                 C                 network.network.network.node   255.255.255.0
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                                                                                                                                            Classless Inter-Domain Routing (CIDR)
                                                                                                                                                  It’s basically the method that ISPs (Internet Service
                                                                                                                                                  Providers) use to allocate an amount of addresses to a
                                                                                                                                                  company, a home—a customer.
                                                                                                                                                  They provide addresses in a certain block size which looks
                                                                                                                                                  something like this: 192.168.10.32/28. This tell you what is
                                                                                                                                                  your subnet mask.
                                                                                                                                                  For example a Class A default subnet mask is 255.0.0.0.
                                                                                                                                                  This means that the first byte of the subnet mask is all ones
                                                                                                                                                  (1s) or 11111111.
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                                                                                                                                            Classless Inter-Domain Routing (CIDR) (contd.)
                                                                                                                                                  When referring to a slash notation, The 255.0.0.0 is
                                                                                                                                                  considered a /8 because it has 8 bits that are 1s—that is, 8
                                                                                                                                                  bits that are turned on.

                                                                                                                                                  For class B default subnet mask is 255.255.0.0, slash
                                                                                                                                                  notation will be /16 and class C default subnet mask is
                                                                                                                                                  255.255.255.0 its slash notation will be /24.

                                                                                                                                                  Refer table 3.2 in Book
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                                                                                                                                            Subnetting Class C Addresses
                                                                                                                                                  In a Class C address, only 8 bits are available for defining the
                                                                                                                                                  hosts. This means that the only Class C subnet masks can be the
                                                                                                                                                  following:
                                                                                                                                            Binary              Decimal Shorthand
                                                                                                                                            ---------------------------------------------------------
                                                                                                                                            10000000 = 128 /25            (Not valid on the Cisco exams!)
                                                                                                                                            11000000 = 192 /26
                                                                                                                                            11100000 = 224 /27
                                                                                                                                            11110000 = 240 /28
                                                                                                                                            11111000 = 248 /29
                                                                                                                                            11111100 = 252 /30
                                                                                                                                            11111110 = 254 /31            (Not valid)
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                                                                                                                                            The Binary Method: Subnetting a Class C Address
                                                                                                                                                  For example, 255.255.255.192., 192 = 11000000
                                                                                                                                                  The 1s represent the subnet bits, and the 0s represent the host bits
                                                                                                                                                  available in each subnet. 192 provides 2 bits for subnetting and 6 bits for
                                                                                                                                                  defining the hosts in each subnet.
                                                                                                                                                  The subnet bits can’t be both off or on at the same time, the only two
                                                                                                                                                  valid subnets are these:
                                                                                                                                                      01000000 = 64 (all host bits off)
                                                                                                                                                      10000000 = 128 (all host bits off)
                                                                                                                                                      The table show Subnet 64

                                                                                                                                                           Subnet      Host           Meaning
                                                                                                                                                           01          000000 = 64    The network (do this first)
                                                                                                                                                           01          000001 = 65    The first valid host
                                                                                                                                                           01          111110 = 126   The last valid host
                                                                                                                                                           01          111111 = 127   The broadcast address (do this second)
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                                                                                                                                            The Fast Way: Subnetting a Class C Address
                                                                                                                                                  How many subnets? 2x – 2 = number of subnets. x is the number
                                                                                                                                                  of masked bits, or the 1s.For example, in 11000000, the number
                                                                                                                                                  of ones gives us 22 – 2 subnets. In this example, there are 2
                                                                                                                                                  subnets.
                                                                                                                                                  How many hosts per subnet? 2y – 2 = number of hosts per
                                                                                                                                                  subnet. y is the number of unmasked bits, or the 0s. For example,
                                                                                                                                                  in 11000000, the number of zeros gives us 26 – 2 hosts. In this
                                                                                                                                                  example, there are 62 hosts per subnet.
                                                                                                                                                  What are the valid subnets? 256 – subnet mask = block size, or
                                                                                                                                                  base number. For example,256 – 192 = 64. 64 is the first subnet.
                                                                                                                                                  The next subnet would be the base number plus itself, or 64 + 64
                                                                                                                                                  = 128, (the second subnet). You keep adding the base number to
                                                                                                                                                  itself until you reach the value of the subnet mask, which is not a
                                                                                                                                                  valid subnet because all subnet bits would be turned on (1s).
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                                                                                                                                            The Fast Way: Subnetting a Class C Address (contd.)

                                                                                                                                                  What’s the broadcast address for each subnet? The
                                                                                                                                                  broadcast address is all host bits turned on, which is the
                                                                                                                                                  number immediately preceding the next subnet.

                                                                                                                                                   What are the valid hosts? Valid hosts are the numbers
                                                                                                                                                  between the subnets, omitting all 0s and all 1s.
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                                                                                                                                            Subnetting Class B Addresses
                                                                                                                                                  The possible Class B subnet masks that we have are lot
                                                                                                                                                  more possible subnets than we do with a Class C network
                                                                                                                                                  address:

                                                                                                                                                        255.255.128.0   (/17)   255.255.255.0 (/24)
                                                                                                                                                        255.255.192.0   (/18)   255.255.255.128 (/25)
                                                                                                                                                        255.255.224.0   (/19)   255.255.255.192 (/26)
                                                                                                                                                        255.255.240.0   (/20)   255.255.255.224 (/27)
                                                                                                                                                        255.255.248.0   (/21)   255.255.255.240 (/28)
                                                                                                                                                        255.255.252.0   (/22)   255.255.255.248 (/29)
                                                                                                                                                        255.255.254.0   (/23)   255.255.255.252 (/30)


                                                                                                                                                  The Class B network address has 16 bits available for host
                                                                                                                                                  addressing. This means we can use up to 14 bits for
                                                                                                                                                  subnetting because we have to leave at least 2 bits for host
                                                                                                                                                  addressing.
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                                                                                                                                            Subnetting Class A Addresses
                                                                                                                                                  Class A subnetting is not performed any differently from Classes B
                                                                                                                                                  and C, but there are 24 bits to play with instead of the 16 in a
                                                                                                                                                  Class B address and the 8 bits in a Class C address.


                                                                                                                                                        255.128.0.0 (/9)      255.255.240.0 (/20)
                                                                                                                                                        255.192.0.0 (/10)     255.255.248.0 (/21)
                                                                                                                                                        255.224.0.0 (/11)     255.255.252.0 (/22)
                                                                                                                                                        255.240.0.0 (/12)     255.255.254.0 (/23)
                                                                                                                                                        255.248.0.0 (/13)     255.255.255.0 (/24)
                                                                                                                                                        255.252.0.0 (/14)     255.255.255.128 (/25)
                                                                                                                                                        255.254.0.0 (/15)     255.255.255.192 (/26)
                                                                                                                                                        255.255.0.0 (/16)     255.255.255.224 (/27)
                                                                                                                                                        255.255.128.0 (/17)   255.255.255.240 (/28)
                                                                                                                                                        255.255.192.0 (/18)   255.255.255.248 (/29)
                                                                                                                                                        255.255.224.0 (/19)   255.255.255.252 (/30)
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                                                                                                                                            Variable Length Subnet Masks (VLSMs)
                                                                                                                                                       A router running RIP has a subnet
                                                                                                                                                       mask of a certain value, it assumes
                                                                                                                                                       that all interfaces within the
                                                                                                                                                       classful address space have the
                                                                                                                                                       same subnet mask. This is called
                                                                                                                                                       classful routing, and RIP and IGRP
                                                                                                                                                       are both considered classful routing
                                                                                                                                                       protocols.
                                                                                                                                                       Classless routing protocols,
                                                                                                                                                       however, do support the
                                                                                                                                                       advertisement of subnet
                                                                                                                                                       information.Therefore, you can use
                                                                                                                                                       VLSM with routing protocols such
                                                                                                                                                       as RIPv2, EIGRP, or OSPF.
                                                                                                                                                       A Typical Classful Network
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                                                                                                                                            VLSM Design
                                                                                                                                                  The mask of 255.255.255.240 (/28) provides 14 subnets,
                                                                                                                                                  each with 14 hosts.All hosts and router interfaces use the
                                                                                                                                                  same subnet mask.




                                                                                                                                                  The WAN links are point-to-point, and use only two IP
                                                                                                                                                  addresses. So we’re basically wasting 12 valid host
                                                                                                                                                  addresses per WAN link.
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                                                                                                                                            VLSM Design (contd.)
                                                                                                                                                  We can use different size masks on each interface. If we do that, we get 2
                                                                                                                                                  hosts per WAN interface and 14 hosts per LAN interface.It makes a huge
                                                                                                                                                  difference, we get more hosts on a LAN, we still have room to add more
                                                                                                                                                  WANs and LANs on the same network.
                                                                                                                                                  By using a VSLM design, we save address space
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                                                                                                                                            Implementing VLSM Networks
                                                                                                                                                  To create VLSMs quickly and efficiently, you need to
                                                                                                                                                  understand how block sizes and charts work together to
                                                                                                                                                  create the VLSM masks.

                                                                                                                                                  Block size for class C Network.

                                                                                                                                                    Prefix             Mask    Hosts Block Size
                                                                                                                                                    /26                       192    62       64
                                                                                                                                                    /27                224    30      32
                                                                                                                                                    /28                       240    14      16
                                                                                                                                                    /29                       248     6       8
                                                                                                                                                    /30                       252     2       4
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                                                                                                            Example of VLSM
                                                                                                                              Implementing VLSM Networks (contd.)
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                                                                                                                                            Troubleshooting IP Addressing
                                                                                                                                                  Here are the four troubleshooting steps Cisco recommends:

                                                                                                                                                    –    ping 127.0.0.1.
                                                                                                                                                    –    ping the IP address of the local host.
                                                                                                                                                    –    ping the default gateway (router).
                                                                                                                                                    –    ping the remote server.

                                                                                                                                                  If steps 1 through 4 were successful, then you know that
                                                                                                                                                  you have IP communication between the local host and the
                                                                                                                                                  remote server. You also know that the remote physical
                                                                                                                                                  network is working.
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                                                                                                                                            Troubleshooting IP Addressing (contd.)
                                                                                                                                                  If the user still can’t communicate with the server after
                                                                                                                                                  steps 1 through 4 are successful, then you probably have
                                                                                                                                                  some type of name resolution problem, and need to check
                                                                                                                                                  your Domain Name Server (DNS) settings.

                                                                                                                                                  If the ping to the remote server fails, then you know you
                                                                                                                                                  have some type of remote physical network problem, and
                                                                                                                                                  need to go to the server and work through steps 1 through
                                                                                                                                                  3 until you find the snag.
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                                                                                                                                            Determining IP Address Problems
                                                                                                                                                     It’s common for a host,
                                                                                                                                                     router, or other network
                                                                                                                                                     device to be configured
                                                                                                                                                     with the wrong IP
                                                                                                                                                     address, subnet mask, or
                                                                                                                                                     default gateway.

                                                                                                                                                     The Fast Ethernet port of
                                                                                                                                                     Lab_B is using Broadcast
                                                                                                                                                     Address of network 64
                                                                                                                                                     which is invalid,change
                                                                                                                                                     the IP Address              192.168.1.97/27   192.168.1.100/27

				
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