166x Filetype PPT File size 0.14 MB Source: www.andrew.cmu.edu
Subnetting It might be useful for the same administrative domain to contain It might be useful for the same administrative domain to contain several different networks: several different networks: Different link layer protocols without complex bridges Different link layer protocols without complex bridges Different administrative subdomains Different administrative subdomains Smaller tables on routers Smaller tables on routers But Class-B network addresses are running out, so we can’t waste But Class-B network addresses are running out, so we can’t waste them by assigning them where they won’t be densely populated them by assigning them where they won’t be densely populated And Class-C network addresses can only accommodate networks off And Class-C network addresses can only accommodate networks off 255 machines, which is too small for even modest networks. 255 machines, which is too small for even modest networks. The solution is to carve sub-networks (“subnets”) out of The solution is to carve sub-networks (“subnets”) out of larger Class-B networks larger Class-B networks Subnetting, cont. If all routers needed to be aware of subnets, this would If all routers needed to be aware of subnets, this would drastically increase the size of routing tables. drastically increase the size of routing tables. To avoid this problem, routers outside of a network only To avoid this problem, routers outside of a network only know about whole networks know about whole networks Routers within a network however, need to know about the Routers within a network however, need to know about the subnets of their network. subnets of their network. This is achieved by having routers inside and outside of a This is achieved by having routers inside and outside of a network interpret the IP address differently. network interpret the IP address differently. Subnetting, cont. For compatibility, routers outside of a particular network must operate For compatibility, routers outside of a particular network must operate “exactly as before” – they make routing decisions based only on the “exactly as before” – they make routing decisions based only on the network number. network number. But, once within a network, the routers are all within the same But, once within a network, the routers are all within the same administrative domain, and can be configured to operate a little administrative domain, and can be configured to operate a little differently differently The portion of the address space that is normally used to identify the The portion of the address space that is normally used to identify the host number is partitioned into a subnet number and host number. This host number is partitioned into a subnet number and host number. This is done in much the same way as the IP address itself was partitioned is done in much the same way as the IP address itself was partitioned into a network number and host number. into a network number and host number. But, unlike the IP address as a whole, the number of bits used for each But, unlike the IP address as a whole, the number of bits used for each is not fixed. Instead it is determined by the subnet mask. is not fixed. Instead it is determined by the subnet mask. Subnetting, cont. Each host can be part of one subnet. This subnet has a subnet number and a Each host can be part of one subnet. This subnet has a subnet number and a subnet mask. subnet mask. For example 128.96.64.0 is a subnet of network 128.96 For example 128.96.64.0 is a subnet of network 128.96 The network number is parsed exactly as it is before, based on the packet’s The network number is parsed exactly as it is before, based on the packet’s prefix and the class of the address. prefix and the class of the address. Within a particular address, the subnet mask is used to separate the subnet Within a particular address, the subnet mask is used to separate the subnet number from the host number. number from the host number. The subnet mask is the same length as an IP address and contains a 1 in every The subnet mask is the same length as an IP address and contains a 1 in every position that is part of the subnet number (including network number) and a 0 position that is part of the subnet number (including network number) and a 0 everywhere else. It might also need to contain additional 1s to distinguish it everywhere else. It might also need to contain additional 1s to distinguish it from other subnets. from other subnets. By ANDing the subnet mask with the IP address, the host number is By ANDing the subnet mask with the IP address, the host number is eliminated, leaving only the subnet number. In this way a host knows if a eliminated, leaving only the subnet number. In this way a host knows if a destination is on its own network. destination is on its own network. Subnetting, cont. Subnets and subnet masks Subnet Num: 128. 96. 0. 0 10000000.01100000.00000000.00000000 Subnet Mask: 255.255.255. 128 11111111.11111111.11111111.10000000 ~127 hosts Notice that the network mask hides host numbers: IP Address: 128. 96. 0. 82 10000000.01100000.00000000.01010010 Subnet Mask: 255.255.255. 128 11111111.11111111.11111111.10000000 Subnet Num: 128. 96. 0. 128 10000000.01100000.00000000.0d0000000
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