IP Subnetting and Variable Length Subnet Masks (VLSMs)
VLSM network, example two
Net = B Net = C 10 hosts 12 hosts
A: /27 B: /28 C: /28 D: /30 E: /30 F: /30 G: /28 H: /26 I: /28 J: /26 K: /28
30 hosts Net = A
2 hosts Net = E
2 hosts Net = F
Fa0/3 2 hosts Net = D
14 hosts 60 hosts
Net = H
Net = I
Net = J
Net = K
12 hosts Net = G
Let’s do another one. Figure 3.7 shows a network with 11 networks, two block sizes of 64, one of 32, five of 16, and three of 4.
First, create your VLSM table and use your block size chart to fill in the table with the sub- nets you need. Figure 3.8 shows a possible solution.
Notice that we filled in this entire chart and only have room for one more block size of 4! Only with a VLSM network can you provide this type of address space savings.
Keep in mind that it doesn’t matter where you start your block sizes as long as you always count from zero. For example, if you had a block size of 16, you must start at 0 and count from there—0, 16, 32, 48, etc. You can’t start a block size of 16 from, say, 40 or anything other than increments of 16.
Here’s another example. If you had block sizes of 32, you must start at zero like this: 0, 32, 64, 96, etc. Just remember that you don’t get to start wherever you want, you must always start counting from zero. In the example in Figure 3.8, I started at 64 and 128, with my two block sizes of 64. I didn’t have a lot of choice, because my options are 0, 64, 128, and 192. However, I added the block size of 32, 16, 8, and 4 wherever I wanted just as long as they were in the cor- rect increments of that block size.
One last example on VLSM design before we move onto summarization. Okay—you have three locations you need to address, and the IP network you have received is 192.168.55.0 to use as the addressing for the entire network. You’ll use ip subnet-zero and RIPv2 as the routing protocol (RIPv2 supports VLSM networks, RIPv1 does not—both of them will be discussed in Chapter 5). Figure 3.9 shows the network diagram and the IP address of the RouterA S0/0 interface.