3 Easy Subnetting

Question 1

What is the subnet mask for a Class A network?

Answer 1

/8
255.0.0.0
Network.Node.Node.Node

Question 2

What is the subnet mask for a Class B network?

Answer 2

/16
255.255.0.0
Network.Network.Node.Node

Question 3

What is the subnet mask for a Class C network?

Answer 3

/24
255.255.255.0
Network.Network.Network.Node

Question 4

What is the “ip subnet-zero” command?

Answer 4

This command allows you to use the first subnet in your network design. For instance, the Class C mask of 255.255.255.192 provides subnets 64,128 and 192

Question 5

What command shows you the routing table on a router?

Answer 5

show ip route

Question 6

What is the only instance you should use a /30 mask and why?

Answer 6

Point to Point link because /30 provides only 2 addresses

Question 7

What is the subnet and broadcast address of the subnet in which 172.16.10.33 /27 resides?

Answer 7

256 – 224 = 32. 32 + 32 = 64. 33 is between 32 and 64. But remember that the third octet is considered part of the subnet, so the answer would be the 10.32 subnet. The broadcast is 10.63, since 10.64 is the next subnet.

Question 8

What subnet and broadcast address is the IP address 172.16.66.10 255.255.192.0 (/18) a member of?

Answer 8

256 – 192 = 64. 0, 64, 128. The subnet is 172.16.64.0. The broadcast must be 172.16.127.255 since 128.0 is the next subnet.

Question 9

What subnet and broadcast address is the IP address 172.16.50.10 255.255.224.0 (/19) a member of?

Answer 9

256 – 224 = 0, 32, 64. The subnet is 172.16.32.0, and the broadcast must be 172.16.63.255 since 64.0 is the next subnet.

Question 10

What subnet and broadcast address is the IP address 172.16.46.255 255.255.240.0 (/20) a member of?

Answer 10

256 – 240 = 16. 0, 16, 32, 48. This subnet address must be in the 172.16.32.0 subnet, and the broadcast must be 172.16.47.255 since 48.0 is the next subnet. So, yes, 172.16.46.255 is a valid host.

Question 11

What subnet and broadcast address is the IP address 172.16.45.14 255.255.255.252 (/30) a member of?

Answer 11

256 – 252 = 0, 4, 8, 12, 16… The subnet is 172.16.45.12, with a broadcast of 172.16.45.15 because the next subnet is 172.16.45.16.

Question 12

What is the subnet and broadcast address of the host 172.16.88.255/20?

Answer 12

A /20 is 255.255.240.0, gives us a block size of 16 in the third octet, and since no subnet bits are on in the fourth octet, the answer is always 0 and 255 in the fourth octet: 0, 16, 32, 48, 64, 80, 96. Because 88 is between 80 and 96, the subnet is 80.0 and the broadcast address is 95.255.

Question 13

A router receives a packet on an interface with a destination address of 172.16.46.191/26. What will the router do with this packet?

Answer 13

Discard it. 172.16.46.191/26 is a 255.255.255.192 mask, which gives us a block size of 64. Our subnets are then 0, 64, 128 and 192. 191 is the broadcast address of the 128 subnet, and by default, a router will discard any broadcast packets.