Which class of IPv4 address provides a maximum of only 254 valid host addresses per network ID in classful addressing?

Introduction / Context:
Although modern networks widely use classless addressing and CIDR, basic classful addressing concepts are still tested in many Cisco certification and networking exams. In classful IPv4 addressing, different classes reserve different numbers of bits for the network and host portions. This question asks which class allows a maximum of 254 host addresses on a single network ID, which is a classic fact related to Class C networks.

Given Data / Assumptions:

We are using classful IPv4 addressing without subnetting within a classful network ID.
We want the class that yields 254 usable host addresses per network ID.
Recall that usable hosts are 2^h minus 2, where h is the number of host bits, subtracting network and broadcast addresses.

Concept / Approach:
In classful addressing, Class A uses an 8 bit network portion and 24 host bits, Class B uses 16 network bits and 16 host bits, and Class C uses 24 network bits and 8 host bits. For a Class C network, the host portion is 8 bits long. The total number of host addresses is 2^8 = 256. Two addresses are reserved: the network address and the broadcast address. Therefore, the number of valid usable host addresses in a Class C network is 256 minus 2, which equals 254. This matches the requirement in the question.

Step-by-Step Solution:
Step 1: Recall the host bits for each class in classful addressing: Class A has 24 host bits, Class B has 16, and Class C has 8.Step 2: Compute usable host addresses as 2^host_bits minus 2 for each class.Step 3: For Class C, 2^8 minus 2 equals 256 minus 2, which is 254 usable hosts.Step 4: Choose Class C as the correct answer because it yields exactly 254 usable host addresses per network ID.

Verification / Alternative check:
As an alternative check, consider the default mask of a Class C network, which is 255.255.255.0. This leaves the last octet, 0 to 255, for host addresses. The address ending with .0 is reserved as the network identifier, and the address ending with .255 is reserved for broadcast. That leaves the range .1 to .254 for host devices, which is 254 addresses in total. This confirms that Class C is the correct answer.

Why Other Options Are Wrong:
Option A, Class A, is wrong because with 24 host bits it supports 2^24 minus 2 hosts, which is far more than 254.
Option B, Class B, is wrong because 16 host bits give 2^16 minus 2 addresses, again much larger than 254 usable hosts.
Option D, Class D, is wrong because Class D is used for multicast, not for unicast host addressing, and does not have a standard count of hosts in the same sense as Classes A, B, and C.

Common Pitfalls:
A common pitfall is to forget that network and broadcast addresses cannot be assigned to hosts, leading to the mistaken conclusion of 256 hosts for a Class C network. Another mistake is to confuse classless subnetting where host counts change with the base classful host counts. Always distinguish between the default classful behavior and any extra subnetting that may be applied on top of it.

Final Answer:
Class C