BARC Data Entry Officer Exam: 10.09.2022প্রশ্ন ও সমাধান

A default subnet mask is the standard mask assigned to each IPv4 address class before any custom subnetting is applied — it marks which bits of the address are the network portion (1s) versus the host portion (0s).

Class	Default Subnet Mask	Network bits	Host bits
A	255.0.0.0	8	24
B	255.255.0.0	16	16
C	255.255.255.0	24	8

• Class A supports few networks with huge numbers of hosts (large ISPs/organisations); Class C supports many networks with few hosts each (typical small LANs).
• Classes D (multicast) and E (experimental/reserved) do not have default subnet masks, since they are not used for conventional network/host addressing.
• Example: IP 192.168.1.10 (Class C) uses the default mask 255.255.255.0, so the first three octets (192.168.1) identify the network and the last octet identifies the host.

Exam takeaway: Class A → 255.0.0.0, Class B → 255.255.0.0, Class C → 255.255.255.0; Classes D/E have no default mask since they are not used for standard host addressing.