Ccna routing and Switching Complete Study Guide

Subnet

0.0

128.0

First host

0.1

128.1

Last host

127.254

255.254

Broadcast

127.255

255.255

Okay, notice that we just added the fourth octet’s lowest and highest values and came 

up with the answers. And again, it’s done exactly the same way as for a Class C subnet. We 

just used the same numbers in the third octet and added 0 and 255 in the fourth octet—

pretty simple, huh? I really can’t say this enough: it’s just not that hard. The numbers never 

change; we just use them in different octets!

Question: Using the previous subnet mask, do you think 172.16.10.0 is a valid host 

address? What about 172.16.10.255? Can 0 and 255 in the fourth octet ever be a valid host 

address? The answer is absolutely, yes, those are valid hosts! Any number between the sub-

net number and the broadcast address is always a valid host.