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Chapter 15
Variable-Length Subnet Masks and Route Summarization
Variable-Length Subnet Masks
- VLSM allows more than one subnet mask to be used within a single Class A, B, or C network.
VLSM allows the creation of efficient network addressing plan by using different subnet masks
for different networks – some subnets to be larger and some to be smaller, which is able to
reduce the wasted IP addresses in each subnet, and hence allows the creation of more subnets.
- A routing protocol that supports VLSM (eg: RIPv2, EIGRP, and OSPF) must be used for routers
to learn VLSM routes in a network. A routing protocol that supports VLSM advertises not only
the subnet number, but also the subnet mask along with its routing updates.
- The most efficient subnet mask for point-to-point links is 255.255.255.252 (/30). It provides 1
network address, 2 host addresses, and 1 broadcast address, which is sufficient for 2 end systems
on a point-to-point link.
- The following table shows the network addresses for the common subnet masks:
Mask Subnet Bits Host Bits Block Size Network Addresses
/24 0 8 256 addresses .0
/25 1 7 128 addresses .0, or .128
/26 2 6 64 addresses .0, .64, .128, or .192
/27 3 5 32 addresses .0, .32, .64, .96, .128, .160, .192, or .224
/28 4 4 16 addresses .0, .16, .32, … (multiplier of 16)
/29 5 3 8 addresses .0, .8, .16, … (multiplier of 8)
/30 6 2 4 addresses .0, .4, .8, … (multiplier of 4)
- Caution: Watch out for the subnets overlap issue when implementing VLSM networks.
10.2.1.0
10.2.2.0
10.2.3.0
10.2.4.0
Figure 15-1: Sample VLSM Network
- Below shows the routing table on RT2 in the sample VLSM network with 2 separate masks (/24
and /30) configured on interfaces, along with autosummarized EIGRP routes from other routers:
RT2#sh ip route
Gateway of last resort is not set
RT1 10.1.2.0/30 RT2 10.1.3.0/30 RT3
S0/0
S0/0
10.2.0.0/16
S0/0
10.1.1.0/24
10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks
D 10.2.0.0/16 [90/2195456] via 10.1.2.1, 00:00:07, Serial0/0
D 10.3.0.0/16 [90/2195456] via 10.1.3.2, 00:06:27, Serial0/1
C 10.1.1.0/24 is directly connected, Ethernet1/0
C 10.1.2.0/30 is directly connected, Serial0/0
C 10.1.3.0/30 is directly connected, Serial0/1
RT2#
Note: EIGRP autosummarization is enabled in this scenario.
111
S0/1
10.3.0.0/16
10.3.4.0
10.3.5.0
10.3.6.0
10.3.7.0
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com