Network FundamentalsDifficulty 3/5
How many usable host addresses are in a /31 subnet on a point-to-point link, and why?
- A0, because network and broadcast consume both addresses
- B2, because RFC 3021 allows both addresses to be assigned on point-to-point links
- C1, because one address is reserved for the broadcast
- D4, because /31 has four addresses
Show answer and rationale
Correct: B. A /31 has two addresses. RFC 3021 permits using both on point-to-point links, since no broadcast address is needed there. Cisco IOS supports this with the ip address ... 255.255.255.254 form.
- A — The classic 2^n − 2 rule does not apply to /31 per RFC 3021.
- C — No broadcast address is reserved on a /31 point-to-point link.
- D — 2^(32−31) = 2 addresses, not 4.