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Table Of Contents  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

Previous Topic/Section
IP Default Subnet Masks For Address Classes A, B and C
Previous Page
Pages in Current Topic/Section
123
4
Next Page
IP Subnet Identifiers, Subnet Addresses and Host Addresses
Next Topic/Section

IP Custom Subnet Masks
(Page 4 of 4)

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.

Key Concept: The number of hosts allowed in each subnet is the binary power of the number of host ID bits remaining after subnetting, less two. The reduction by two occurs because the all-zeroes and all-ones host IDs within each subnet are reserved for two “special meaning” addresses: to refer to the subnetwork itself and its local broadcast address. In some implementations, the number of subnets is also reduced by two because the all-zeroes and all-ones subnet IDs were originally not allowed to be used.


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.

Note: In this Guide I assume we are dealing with modern hardware and do not exclude the all-zeroes and all-ones subnets, but I do try to make explicit note of this fact wherever relevant.



Previous Topic/Section
IP Default Subnet Masks For Address Classes A, B and C
Previous Page
Pages in Current Topic/Section
123
4
Next Page
IP Subnet Identifiers, Subnet Addresses and Host Addresses
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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