Cisco CCNA (200-301)Network FundamentalsHard
A network engineer must assign IP addressing for a serial WAN link connecting two routers point-to-point. To conserve address space, which subnet mask should be used for this link?
- A/24, providing 254 usable addresses
- B/30, providing 2 usable addresses
- C/28, providing 14 usable addresses
- D/31, providing 256 usable addresses
Show answer & explanationAnswer & explanation
Correct answer: B. /30, providing 2 usable addresses
A point-to-point WAN link only requires two usable IP addresses, one for each router interface. A /30 subnet provides exactly 2^(32-30)-2 = 2 usable host addresses, making it the most efficient standard subnet choice for this scenario.
Why the other options are wrong
- A. A /24 wastes 252 addresses on a link that only needs 2, which is inefficient.
- C. A /28 provides 14 usable addresses, still far more than needed for two routers.
- D. /31 actually provides only 2 usable addresses under RFC 3021 (no network/broadcast reserved), not 256; this option's math is incorrect.
Point-to-Point Subnetting (/30)
A /30 subnet is commonly used for WAN links between two routers because it provides exactly 2 usable host addresses, minimizing wasted address space.
- /30 = 4 total addresses, 2 usable (network + broadcast reserved)
- /31 (RFC 3021) can also be used, providing 2 usable addresses with no reserved network/broadcast
- Larger subnets waste addresses on point-to-point links
Memory trick: 'Two routers, two addresses — /30 fits like a glove.'