TCP/IP Tutorial and Technical Overview - IBM Redbooks

Static subnetting example
Consider the Class A network shown in Figure 3-2.
Figure 3-2 IP: Class A address without subnets
Use the IP address shown in Figure 3-3.
Figure 3-3 IP address
1 2 3
01 8 6 4 1
0
Class A netID hostID
00001001 01000011 00100110 00000001 a 32-bit address
The IP address is 9.67.38.1 (Class A) with 9 as the and
67.38.1 as the .
The network administrator might want to choose the bits from 8 to 25 to indicate
the subnet address. In that case, the bits from 26 to 31 indicate the host
addresses. Figure 3-4 shows the subnetted address derived from the original
Class A address.
Class A
Subnet
9 67 38 1 decimal notation (9.67.38.1)
1 2 3
01 8 6 4 1
0
netID
subnet number
Figure 3-4 IP: Class A address with subnet mask and subnet address
host
ID
A bit mask, known as the subnet mask, is used to identify which bits of the
original host address field indicate the subnet number. In the previous example,
the subnet mask is 255.255.255.192 (or 11111111 11111111 11111111
11000000 in bit notation). Note that, by convention, the is
included in the mask as well.
Chapter 3. Internetworking protocols 75