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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Subnet Addressing ("Subnetting") Concepts

IP Default Subnet Masks For Address Classes A, B and C 1 2 3 4 IP Subnet Identifiers, Subnet Addresses and Host Addresses

Subtracting Two From the Number of Hosts Per Subnet and (Possibly) Subnets Per Network

There's one more issue that needs to be explained regarding the split into subnet ID and host ID. We've already seen how in regular “classful” addressing, we must subtract 2 from the number of hosts allowed in each network. This is necessary because two host IDs in each subnet have “special meanings”: the all-zeroes host ID meaning “this network”, and the all-ones host ID which is a broadcast to “all hosts on the network”. These restrictions apply also to each subnet under subnetting too, which is why we must continue to subtract 2 from the number of hosts per subnet. (This is also why dividing the 8 host ID bits of a Class C network into 7 bits for subnet ID and 1 bit for host ID is not just silly, but in fact meaningless: it leaves 2¹-2=0 hosts per subnet. Not particularly useful.)

There is a similar issue that occurs with the subnet ID as well. When subnetting was originally defined in RFC 950, the standard specifically excluded the use of the all-zeroes and all-ones subnets. This was due to concern that routers might become confused by these cases. A later standard, RFC 1812 (Requirements for IP Version 4 Routers) removed this restriction in 1995. Thus, modern hardware now has no problem with the all-zeroes or all-ones subnets, but some very old hardware may still balk at it.

For this reason, you will sometimes see discussions of subnetting that exclude these cases. When that is done, you lose 2 potential subnets: the all-zeroes and all-ones subnets. If you do this, then choosing 1 bit for subnet ID is no longer valid, as it yields 2¹-2=0 subnets. You must choose 2 bits if you need 2 subnets.

IP Default Subnet Masks For Address Classes A, B and C 1 2 3 4 IP Subnet Identifiers, Subnet Addresses and Host Addresses