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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
1
2
34
Next Page
IP Subnet Identifiers, Subnet Addresses and Host Addresses
Next Topic/Section

IP Custom Subnet Masks
(Page 2 of 4)

Trading Off Bit Allocations To Meet Subnetting Requirements

How do we decide how to divide the “classful” host ID into subnet ID and host ID bits? This is the key design decision in subnetting. We must make this choice based on our requirements for the number of subnets that exist in the network, and also on the maximum number of hosts that need to be assigned to each subnet in the network. For example, suppose we have 10 total subnets for our Class B network. We need 4 bits to represent this, because 24 is 16 while 23 is only 8. This leaves 12 bits for the host ID, for a maximum of 4,094 hosts per subnet.

However, suppose instead that we have 20 subnets. If so, 4 bits for subnet ID won't suffice: we need 5 bits (25=32). This means in turn that we now have only 11 bits for the host ID, for a maximum of 2,046 hosts per subnet. Step #2 of the practical subnetting example discusses these decisions in more detail.

Now, what happens if we have 20 subnets and also need a maximum of 3,000 hosts per subnet? Well, we have a problem. We need 5 bits to express 20 different subnets. However, we need 12 bits to express the number 3,000 for the host ID. That's 17 bits—too many. The solution? We might be able to shuffle our physical networks so that we only have 16. If not, we need a second Class B network.

It's also important to realize that in regular subnetting, the choice of how many bits to use for the subnet ID is fixed for the entire network. You can't have subnets of different sizes—they must all be the same. Thus, the number of hosts in the largest subnet will dictate how many bits you need for the host ID. This means that in the case above, if you had a strange configuration where 19 subnets had only 100 hosts each but the 20th had 3,000, you'd have a problem. If this were the case, you could solve the problem easily by dividing that one oversized subnet into two or more smaller ones. An enhancement to subnetting called Variable Length Subnet Masking (VLSM) was created in large part to remove this restriction.

Note: I have included summary tables that show the trade-off in subnetting each of Classes A, B and C, and the subnet mask for each of the choices.


Key Concept: The fundamental trade-off in subnetting: each addition of a bit to the subnet ID (and thus, subtraction of that bit from the host ID) doubles the number of subnets, and approximately halves the number of hosts in each subnet. Each subtraction of a bit from the subnet ID (and addition of that bit to the host ID) does the opposite.



Previous Topic/Section
IP Default Subnet Masks For Address Classes A, B and C
Previous Page
Pages in Current Topic/Section
1
2
34
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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