VLSM in Practice

With the Variable Length Subnet Mask (VLSM), the LAN and WAN segments can be assigned addresses without any waste. As shown the scenarios in Figure 1, the hosts in each of the sub-network will be assigned a valid host address with the range of that subnet and /26 masks. Each of the routers will have a LAN interface with a /26 subnet and serial interfaces with a /30 subnet.

 

Using the addressing scheme we already discussed in the previous lesson, the first host IPv4 address for each subnet is assigned to the LAN interface of the router. The WAN interfaces of the routers are assigned the IP addresses and mask for the /30 subnets. Hosts on each subnet will have a host IPv4 address from the range of host addresses for that subnet and an appropriate mask. Hosts will use the address of the attached router LAN interface as the default gateway address. The table below is the 

 

addressing scheme we already discussed in the previous lesson.

Subnet	Network ID	Remarks
0	130.10.0.0/26	Assign to LAN-1
1	130.10.0.64/26	Assign to LAN-2
2	130.10.0.128/26	Assign to LAN-3
3	130.10.0.192/26	Assign to LAN-5
4	130.10.1.0/26	Not used /Spare for future expansion
5	130.10.1.64/26	Not used /Spare for future expansion
6	130.10.1.128/26	Not used /Spare for future expansion
7	130.10.1.192/27	Assign to LAN-4
8	130.10.1.224/30	Assign to WAN R1-R2
9	130.10.1.228/30	Assign to WAN R2-R3
10	130.10.1.232/30	Assign to WAN R3-R4
11	130.10.1.236/30	Assign to WAN R4-R5
12	130.10.1.240/30	Not used /Spare for future expansion of WAN
13	130.10.1.244/30	Not used /Spare for future expansion of WAN
14	130.10.1.248/30	Not used /Spare for future expansion of WAN
15	130.10.1.252/30	Not used /Spare for future expansion of WAN

 

 

VLSM Chart

An addressing chart is an important element to identify which blocks of addresses are already used which block are available. as shown in the above table. The VLSM chart helps to avoid assigning addresses that have previously been allocated.