Information Technology Reference
In-Depth Information
With fixed-length subnet masks, the network supports only 16 networks. Any LAN or
WA N l i n k h a s t o u s e a / 2 0 s u b n e t . I n t h i s s c e n a r i o , i f t h e s i t e s i n v o l v e d v a r y i n s i z e , t h i s
“one network size fits all” solution might be a waste of address space and therefore is inef-
ficient. With VLSMs, you can further subnet the /20 subnets.
For example, take 130.20.64.0/20 and subdiv ide it to suppor t L ANs w ith about 500 hosts.
A /23 mask has 9 bits for hosts, producing 2
9
- 2 = 510 IP addresses for hosts. Table 8-21
shows the subnetworks for LANs within a specified subnet.
Ta b l e 8 - 2 1
Subnetworks for 130.20.64.0/20
Third Byte
Subnetwork
0100
000
0
130.20.64.0/23
0100
001
0
130.20.66.0/23
0100
010
0
130.20.68.0/23
0100
011
0
130.20.70.0/23
0100
100
0
130.20.72.0/23
0100
101
0
130.20.74.0/23
0100
110
0
130.20.76.0/23
0100
111
0
130.20.78.0/23
With VLSMs, you can further subdivide these subnetworks of subnetworks. Take subnet-
work 130.20.76.0/23 and use it for two LANs that have fewer than 250 hosts. It produces
subnetworks 130.20.76.0/24 and 130.20.77.0/24. Also, subdivide 130.20.78.0/23 for serial
links. Because each point-to-point serial link needs only two IP addresses, use a /30 mask.
Ta ble 8 - 2 2 shows the subnetworks produced.
Ta b l e 8 - 2 2
Serial-Link Subnetworks
Third Byte
Fourth Byte
Subnetwork
01001110
000000
00
130.20.78.0/30
01001110
000001
00
130.20.78.4/30
01001110
000010
00
130.20.78.8/30
01001110
000011
00
130.20.78.12/30
. . .
. . .
. . .
01001111
111101
00
130.20.79.244/30
01001111
111110
00
130.20.79.248/30
01001111
111111
00
130.20.79.252/30
