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Troubleshooting IP Addressing

147

FIGURE 3.15

Find the valid host #2

RouterA

RouterB

E0: 192.168.10.65/26

E0: 192.168.10.33/28

HostA

HostB

FIGURE 3.16

Find the valid host address #3

172.16.17.0/22

RouterA

RouterB

S0/0

S0/0

First, you must know that a /22 CIDR is 255.255.252.0, which makes a block size of 4 in the third octet. Since 17 is listed, the available range is 16.1 through 19.254; so, for example, the IP address S0/0 could be 172.16.18.255 since that’s within the range.

Okay, last one! You have one Class C network ID, and you need to provide one usable sub- net per city, while allowing enough usable host addresses for each city specified in Figure 3.17. What is your mask?

Actually, this is probably the easiest thing you’ve done all day! I count five subnets needed and the Wyoming office needs 16 users (always look for the network that needs the most hosts). What block size is needed for the Wyoming office? 32. (Remember, you cannot use a block size of 16 because you always have to subtract 2!) What mask provides you with a block size of 32? 224. Bingo! This provides 8 subnets, each with 30 hosts.

FIGURE

3.17

Find the valid subnet mask

Corporate 7 users

L.A.

S.F.

N.Y.

Wy.

15 users

13 users

7 users

16 users

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