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What is the function of the SUBNET MASK in IPv4?
AEncrypts the IP address
BSeparates the network portion from the host
CCounts the number of hops to a destination
DSelects which protocol to use
Answer & Solution
Correct answer: B. Separates the network portion from the host
1. An IPv4 address is divided into NETWORK and HOST portions. The subnet mask says where the division is.
2. Example: address 192.168.1.42 with mask 255.255.255.0 (or /24):
- The first 24 bits (192.168.1) is the NETWORK identifier.
- The last 8 bits (42) is the HOST within that network.
3. The mask consists of $1$-bits for the network portion, $0$-bits for the host portion (always contiguous).
4. CIDR notation /24 means 24 network bits. Common masks: /16, /24, /28, /30 for point-to-point links.
5. Other options misstate the role.
_Source: RFC 1180 "A TCP/IP Tutorial", §5 (IP — Addressing and Subnetting)._
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