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# CIDR

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```									Supernetting

Lecturer - MM Clements
This week …..

   Address aggregation and ISPs
   IPv4 address allocation
   Classful Addressing
   Supernetting
IPv4 classful addressing

   32 bit address
   How many do we get?
   ½ are class A
   ¼ are class B
   1/8 are class C
   Rest are D & E
Wastage using classes rigidly

   232 = 4 294 967 296
   ½ of these are class A = 2 147 483 648
   Class A only given to big organisations
–   Governments etc
   1- 126 (10.x.x.x excepted – why?)
   Each class A has (224 –2) hosts = 16 777 216
   A lot of hosts!!
Class B Allocations

   216 = 65536 hosts per class B
   1 073 741 824 addresses in total
   16 384 different class B addresses
   128.0.0.0 to 191.255.255.255
   Given to large companies
   A few lost 172.16.0.0 to 172.31.255.255 –
why?
Class C Allocations

   536 870 912 host addresses in all
   28 –2 = 254 hosts per class C address
   192.0.0.0 to 223.255.255.255
   Can’t use 192.168.x.x – why?
   2 097 152 class C addresses available
   Not large enough for big company, too many
for small company.
Internet Core Routing

   Consider core router lookup table
   Over 2M lines to sift through
   How long to look up any one random item on
this table?
   How many cycles per second?
   Estimate this
Solution – longer addresses

   IPv6 created
   Bring in work-arounds
   NAT/ PAT and CIDR
   Launch IPv6 around 2010
   128 – bit address space
   How many addresses per sq metre of Earth?
   Work it out yourself
Classless Interdomain Routing

   1993 (see RFCs 1517, 1518, 1519, 1520 )
   CIDR helps keep Internet from running out of IP
addresses
   Allocates only the amount of address space needed
   Wastage of address space reduced.
   Supernetting - classful subnet masks extended so
that a network address and subnet mask can specify
multiple Class C subnets with one address
   Classless routing
Example

   I have a company and need 1800 IP
addresses ( A, B or C? NO!)
   211 = 2048 (don’t forget to lose 2 of them)
   11 host bits needed
   Mask will be 32 – 11 = 21 i.e. /21
   ISP can allocate this space
   Appears in ISP routing table as 1 entry
   Advertised back to Internet core as 1 address
IP Allocation by ISPs

   Big ISPs are allocated large chunks of
address space
   ISP's customers are then allocated networks
from the big ISP's pool
   ISP has 155.100.0.0 to 155.255.255.255
   10011011. 01100100. 00000|000. 00000000
   Divided as network| hosts
Example

   10011011. 01100100. 00000|000. 00000000
   Network address
   10011011. 01100100. 00000|111. 11111111
   Broadcast address (without subnetting)
   Convert to decimal
   155.100.0.0 /21 to 155.100.7.255 /21
   Customer can subdivide as sees fit
Another Example – 8 class Cs

   200.10.24.0 = 11001000. 00001010. 00011000. 00000000
   200.10.25.0 = 11001000. 00001010. 00011001. 00000000
   200.10.26.0 = 11001000. 00001010. 00011010. 00000000
   200.10.27.0 = 11001000. 00001010. 00011011. 00000000
   200.10.28.0 = 11001000. 00001010. 00011100. 00000000
   200.10.29.0 = 11001000. 00001010. 00011101. 00000000
   200.10.30.0 = 11001000. 00001010. 00011110. 00000000
   200.10.31.0 = 11001000. 00001010. 00011111. 00000000
   Note the common bits in green
   Mask is therefore /21
   200.10.24.0 /21 is the aggregate address
Advertising Upstream

   200.10.24.0 /21 advertised back upstream
   ISP itself will advertise all of its addresses
aggregated too
   Work this out yourself
   Keeps core routing tables manageable
Conclusion ……..

   Poor visions of future led to bad IP allocation
   IP addresses began to run out
   CIDR created to bridge transition to IPv6
   Now working classless
   Uses /number for subnet mask
   Allows for customisation of address space
   Keeps core Internet routing tables
manageable

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