Extending IP Addresses
77
Referring to our table, we can easily determine the subnet mask required
for each of the segments in our example, as shown in Table 3.8.
3.
Starting with the segment requiring the greatest number of subnet bits,
begin allocating addresses.
Let's begin with the serial link, which has 30 bits of subnetting. Since all
of our addresses are going to start with 172.16, we will examine only the last
16 bits of the IP address. In Table 3.9, we see the subnet mask, in binary and
the first and last IP number in the range. Remember that the host portion of
the address cannot be all ones (which is the broadcast address) or all zeros
(which is the address of the network, or wire).
T A B L E 3 . 8
Subnet Masks for Figure 3.2
Description of
Segment
Number of IP
Addresses Required
Subnet Mask (Number
of Subnet Bits)
Server farm
300
255.255.254.0 (23)
Ethernet user segment
200
255.255.255.0 (24)
Serial link
2
255.255.255.252 (30)
Router interconnection
switched segment
4
255.255.255.248 (29)
T A B L E 3 . 9
Networks, Hosts, and Subnets for Figure 3.2
3rd Octet
4th Octet
Decimal IP Address
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
(Last 16 bits in bold)
Subnet Mask
1 1 1 1 1 1 1 1
1 1 1 1 1 1 0 0
255.255.255.252
Network
0 0 0 0 0 0 0 0
0 0 0 0 0 1 0 0
172.16.0.4
First IP in
range
0 0 0 0 0 0 0 0
0 0 0 0 0 1 0 1
172.16.0.5
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com