You are working with an Internet Service Provider (ISP) as network manager. A corporate client approaches
you to lease a public IP subnet that can accommodate 250 users. You have assigned him the 192.25.27.0
subnet.What subnet mask should be assigned to this IP address so that it can accommodate the number of users
required by the corporate client?

A.
255.255.255.0

B.
255.255.255.128

C.
255.255.255.224

D.
255.255.255.252

Explanation:
The 192.25.27.0 subnet should be assigned the subnet mask of 255.255.255.0 to accommodate 250 users.
This subnet mask can accommodate a maximum of 254 hosts. The number of hosts that can reside on a
subnet can be calculated using the formula 2n – 2 = x, where n is equal to the number of hosts bits in the mask
and x is the resulting number of hosts. 2 is subtracted from the results to represent the two address, the
network ID and the broadcast address, that cannot be assigned to computers in the subnet. Since the
255.255.255.0 mask leaves 8 bits at the end of the mask, the formula will be 28 – 2, which is 256 – 2, which
equals 254.
In situations where the same subnet mask must be used for multiple interfaces on a router, the subnet mask
that is chosen must provide capacity sufficient for the largest number of hosts on any single interface while also
providing the required number of subnets. For example, in the diagram below, the three interfaces on the router
R2 have 16, 32 and 58 users respectively on each interface:

If each interface must have the same subnet mask, the subnet mask would need to be one that yields at least
58 addresses to support the interface with the highest host count and yields at least 3 subnets as well.
If the chosen classful networks were 128.107.4.0/24, the correct mask would be 255.255.255.192. Since the
mask is currently 255.255.255.0 (/24), by borrowing 2 bits to /26 or 255.255.255.192, we will get 4 subnets (22
= 4) and each subnet will yield 62 hosts (26 – 2 = 62).
With a subnet mask of 255.255.255.128, the 192.25.27.0 subnet can accommodate only 126 hosts. The mask
255.255.255.128 leaves 7 host bits in the mask and when we plug that into the formula we get 27 – 2, which
equals 126.
With a subnet mask of 255.255.255.224, the 192.25.27.0 subnet can accommodate only 30 hosts. The mask
255.255.255.224 leaves 5 host bits in the mask and when we plug that into the formula we get 25 – 2, which
equals 30.With a subnet mask of 255.255.255.252, the IP address 192.25.27.24 can accommodate only two hosts. The
mask 255.255.255.252 leaves 2 host bits in the mask and when we plug that into the formula we get 22 – 2,
which equals 2.
Objective:
Network Fundamentals
Sub-Objective:
Apply troubleshooting methodologies to resolve problems

Cisco > Design Tech Notes > IP Routing > IP Addressing and Subnetting for New Users > Understanding IP
Addresses > Document ID: 13788