Information Technology Reference
In-Depth Information
Variable-Length Subnet Masks
Va r i a ble - len g t h s u bne t m a s k s (V L S M ) d i v ide a ne t wor k i n to s u bne t s of v a r iou s s i z e s to
prevent wasting IP addresses. If a Class C network uses 255.255.255.240 as a subnet mask,
16 subnets are available, each with 14 IP addresses. If a point-to-point link needs only 2 IP
addresses, 12 IP addresses are wasted. This problem scales further with Class B and Class
A address space. With VLSMs, small LANs can use /28 subnets with 14 hosts, and larger
LANs can use /23 or /22 masks with 510 and 1022 hosts, respectively. Point-to-point net-
works use a /30 mask, which supports two hosts.
Key
To p i c
There is not one way to subdivide a network, so there is not a single correct way to create
subnets. The best practice is to divide large networks into smaller subnets that can be as-
signed to sites. Further divide each site subnet into smaller subnets for data, VoIP, wireless
LAN, and other subnets to be used in site VLANs. Furthermore, WAN and point-to-point
links, router, and switch loopback addresses are allocated IP subnets.
VLSM Address Assignment: Example 1
Let's look at a VLSM IP address assignment example. Take Class B network 130.20.0.0/16
as an example. Using a /20 mask produces 16 subnetworks. Table 8-20 shows the subnet-
works. With the /20 subnet mask, the first 4 bits of the third byte determine the subnets.
Ta b l e 8 - 2 0
Subnets with the /20 Mask
Third Byte
Subnetwork
0000
0000
130.20.0.0/20
0001
0000
130.20.16.0/20
0010
0000
130.20.32.0/20
0011
0000
130.20.48.0/20
0100
0000
130.20.64.0/20
0101
0000
130.20.80.0/20
0110
0000
130.20.96.0/20
0111
0000
130.20.112.0/20
1000
0000
130.20.128.0/20
1001
0000
130.20.144.0/20
1010
0000
130.20.160.0/20
1011
0000
130.20.176.0/20
1100
0000
130.20.192.0/20
1101
0000
130.20.208.0/20
1110
0000
130.20.224.0/20
1111
0000
130.20.240.0/20
