Suppose we want to divide a class-C, 200.0.0.0/24 network into five subnets of t
ID: 3603473 • Letter: S
Question
Suppose we want to divide a class-C, 200.0.0.0/24 network into five subnets of the following nominal sizes:
A: 65=27 hosts
B: 32=25 hosts
C: 32=25 hosts
D: 32=25 hosts
E: 16=24 hosts
Propose subnet-bit values to be assigned for each network. The subnets should not “overlap”
subnet
# host bits needed
# net bits
in last byte
total net bits
subnet bits (each entry 0 or 1)
A
7
8-7=1
/25
___
B
5
8-5=3
/27
___ ___ ___
C
5
8-5=3
/27
___ ___ ___
D
5
8-5=3
/27
___ ___ ___
E
4
8-4=4
/28
___ ___ ___ ___
subnet
# host bits needed
# net bits
in last byte
total net bits
subnet bits (each entry 0 or 1)
A
7
8-7=1
/25
___
B
5
8-5=3
/27
___ ___ ___
C
5
8-5=3
/27
___ ___ ___
D
5
8-5=3
/27
___ ___ ___
E
4
8-4=4
/28
___ ___ ___ ___
Explanation / Answer
subnetting in the real world is where an Internet Service Provider (ISP) has numerous residential/small business customers that only need to have access to Internet sites, such as Web sites, email server provided by the ISP, DNS server(s) provided by the ISP, etc. In this case, each customer only needs a single IP address, so a Class C network, such as 200.0.0.0/24 can be further subdivided into subnets of /30 with a subnet mask of 255.255.255.252, providing 64 subnets with a network address, broadcast address, and 2 host addresses. Since these are point-to-point networks, there is really no need for the network or broadcast addresses, so the ISP will sometimes use a /31 (255.255.255.254 subnet mask) network to serve 128 customers from a single Class C network. (In an actual real-world setting the IP addresses for these customers would probably be dynamically assigned, but this example may help illustrate how a network can be divided into a number of small subnets.)
Another example is where a small/medium sized company is assigned a Class C network that they need to use for their networks at several branch offices, as well as for their larger corporate office.
Let's say that the corporate HQ office has 40 computers, including servers, branch A has 10 computers, branch B has 20 computers, and branch C has 5 computers.
Using the same 200.0.0.0/24 Class C network:
This division leaves quite a number of IP addresses unassigned to any subnet, that could be used if additional branch offices were added in the future, but leaves little or no room for expansion at the branch offices.
You would probably want to consider how much growth is expected in each office, as well, allocating a larger subnet to each office to permit expansion. This will limit the IP addresses available for use at future additional branch offices.
Subnetting is a trade-off - you can trade a larger number of small subnets for a smaller number of larger subnets. Which option is used will depend on the structure of the organization - for an exam question, the scenario for the question will determine which option better fulfils the design requirements of the network.