RFC 917 Philippe Bissohong Internet Subnet Subnet is essentially the break down of one network. Organizations decide for technical reason to split one network into multiple instead of getting many network numbers. This RFC discuss the use of subnets, improvements and problems in routing particularly. The original view of the internet was a two-level hierarchical: the catanet (collection of connected internet networks) and below it, internet network numbers. But it was later realized that it would be insufficient especially because big organizations (Ex: universities or big companies) would need more than one LAN cable to cover an area. So they added a third level, which divide one network into sub-networks. When a company is faced with the problem of assigning internet address it has 3 choices. The first one is acquiring distinct internet number for each cable but the disadvantage is that it creates a really big routing table that is useless outside the organization. The second thing it can do is use one internet network numbers but randomly assign host number, the disadvantage for this one is that it will require that all technology support ARP protocols or broadcasting but not all do. The third choice is subnet; the only problem with subnet is, it changes the internet protocol, so the way IP is implemented now must be change but it is a minor inconvenience with a good improvement to the problem. They propose in this rfc to use a standard internet address format over a variable or self- encoding one. This is their proposal: <network-number><subnet-number><host-number> Where subnet number is constant for a given network, zero indicates no subnet. They also propose that organization will choose subnet field width that is a multiple of 8 but implement as well something that can handle any other width. They reject “recursive subnet” or sub-subnet because of its complexity and the lack of bits in IP address. Broadcasting should not only recognize its own host addresses but also three other kinds of broadcast address: Physical network: all ones (255.255.255.255) will only send in the local physical network Specific network: Contain actual network number while the rest is all ones. Specific subnet: Contain actual network number and subnet number; all ones for host. In order for a host to know the subnet field width in the network in which it is connected or find a gateway on its local network or even find its own address, information needs to be either “hardwired” or broadcast. Hardwired saves information in a disk file but it is not likeable enough. As an alternative, since the majority of LAN technology supports broadcasting, it is better for the host to just send a request for the information. For that reason, they want to extend ICMP protocol, adding “Address format request” and “Address format reply” to ICMP message types.
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