IP Address & Subnetting Questions

Note: If you are not sure about Subnetting, please read our Subnetting Tutorial – Subnetting Made Easy.

Question 1

Explanation

A /30 subnet means subnet mask of 255.255.255.252. But 10.2.1.3 255.255.255.252 is a broadcast IP address; only 209.165.201.2/30 is the usable IP address.

Question 2

Question 3

Explanation

This question asks about the private ranges of IPv4 addresses. The private ranges of each class of IPv4 are listed below:

Class A private IP address ranges from 10.0.0.0 to 10.255.255.255
Class B private IP address ranges from 172.16.0.0 to 172.31.255.255
Class C private IP address ranges from 192.168.0.0 to 192.168.255.255

Only the network 172.28.0.0/16 belongs to the private IP address (of class B).

Question 4

Question 5

Explanation

We need a subnet with 20 users so we need 5 bits 0 in the subnet mask as 2⁵ – 2 = 30 > 20. Therefore the subnet mask should be /27 (with last octet is 1110 0000 in binary). The increment is 32 so the valid network address is 10.10.225.32.

Question 6

Explanation

We see the maximum number of user per floor is 29 users (Floor 2) < 32 so the best subnet mask should be 1110 0000 which allows 2⁵ – 2 = 30 hosts per subnet.

Question 7

Explanation

Each interface on a router must be in a different network. If two interfaces are in the same network, the router will not accept it and show error when the administrator assigns it.

Question 8

Question 9

Explanation

We cannot use the subnet mask of 255.255.254.0 (/23) because it is smaller than the default subnet mask of class C (/24). If we use /23 subnet mask, in fact we are summarizing (supernet) it -> Answer B is not correct.

With the subnet mask of 255.255.255.128, we have 2¹ = 2 subnets only, not enough for 8 floors -> Answer D is not correct.

We need 40 users per subnet so our subnet must support at least 64 (=2⁶) hosts (in fact 62 hosts). So the last octet of subnet mask should be 1100 0000 (with 6 bits “0”) = 192 -> The suitable subnet mask is 255.255.255.192. But this subnet mask only provides 2² = 4 subnets, which is not enough for 8 floors.

For 8 floors we need 1110 0000 (2³ = 8) for the last octet of subnet mask so the subnet mask should be 255.255.255.224.

But with the subnet mask of 255.255.255.224, the host addresses per subnet is 2⁵ – 2 = 30 so we cannot fulfill the host addresses for 40 users. We can only provide IPs for 30 users. This question said “approximately 30-40 users per floor” so “30 users per floor” is acceptable.

With “192.168.0.0 255.255.255.224” we have 8 subnets:
+ First subnet: 192.168.0.0 to 192.168.0.31
+ Second subnet: 192.168.0.32 to 192.168.0.63
+ Third subnet: 192.168.0.64 to 192.168.0.95
+ Fourth subnet: 192.168.0.96 to 192.168.0.127
+ Fifth subnet: 192.168.0.128 to 192.168.0.159
+ Sixth subnet: 192.168.0.160 to 192.168.0.191
+ Seventh subnet: 192.168.0.192 to 192.168.0.223
+ Eighth subnet: 192.168.0.224 to 192.168.0.255