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Chapter 3

Internet Protocols

Question: What subnet and broadcast address is the IP address
172.16.10.10 255.255.255.192 a member of?

Answer: 256

- 192 = 64. This address must be in the 172.16.10.0 sub-

net, and the broadcast must be 172.16.10.63.

Question: What subnet and broadcast address is the IP address
172.16.10.10 255.255.255.252 a member of?

Answer: 256

- 252 = 4. The subnet is 172.16.10.8, with a broadcast

of 172.16.10.11.

Subnetting Class A Addresses

Class A subnetting is not performed any differently from Classes B and C,
but there are 24 bits to play with instead of the 16 in a Class B address and
the eight bits in a Class C address.

Let's start by listing all the Class A subnets:

255.128.0.0 (/9) 255.255.240.0 (/20)

255.192.0.0 (/10) 255.255.248.0 (/21)

255.224.0.0 (/11) 255.255.252.0 (/22)

255.240.0.0 (/12) 255.255.254.0 (/23)

255.248.0.0 (/13) 255.255.255.0 (/24)

255.252.0.0 (/14) 255.255.255.128 (/25)

255.254.0.0 (/15) 255.255.255.192 (/26)

255.255.0.0 (/16) 255.255.255.224 (/27)

255.255.128.0 (/17) 255.255.255.240 (/28)

255.255.192.0 (/18) 255.255.255.248 (/29)

255.255.224.0 (/19) 255.255.255.252 (/30)

That's it. You must leave at least two bits for defining hosts. And I hope you

can see the pattern by now. Remember, we're going to do this the same way
as a Class B or C subnet. It's just that, again, we simply have more host bits.

Subnetting Practice Examples: Class A Addresses

When you look at an IP address and a subnet mask, you must be able to dis-
tinguish the bits used for subnets from the bits used for determining hosts.
This is imperative. If you're still struggling with this concept, please reread
the preceding "IP Addressing" section. It shows you how to determine the
difference between the subnet and host bits, and should help clear things up.

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