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278 Appendices
Chart 1: IP Address Classes
You can always tell what class an address is by looking at the first octet and
comparing it to the chart above. For instance, the address at the top of this
appendix has 198 as the first octet, so it is Class C.
Subnet Masks
A subnet mask tells a router how much of an address it should treat as the
network portion. The masks for traditional Class A, B and C networks are
shown below.
Chart 2: Standard IP Subnets
Comparing the masks above to the first chart, you can see that the 255’s in a
mask identify the network portion of the address.
Just as the masks above specify what portion of the global IP address range a
network is using, a subnet mask can also be used to subdivide a Class A, B or
C network range into multiple groups of hosts, or “subnets.”
This is done by telling the router that more than the traditional number of bits
in the mask are to be treated as the network portion of the address. The chart
below shows all of the possible Class C subnet masks, and how many hosts
are then allowed on each subnet.
C
LASS NETWORK PORTION HOSTS ALLOWED
A from 1.
to 127.
about 16 million
B from 128.0
to 191.255
65,536
C from 192.0.0
to 223.255.255
255
CLASS SUBNET MASK
A 255.0.0.0
B 255.255.0.0
C 255.255.255.0