Which of the following is a characteristic of a subnet created with a /31 prefix?

A /31 prefix, as defined by RFC 3021, is used for point-to-point links to conserve IPv4 address space. A /31 subnet mask leaves only one bit for the host portion, yielding two possible addresses. Instead of designating these as network and broadcast addresses (which would leave no usable host addresses), RFC 3021 allows both addresses to be assigned to the two endpoints of the link. Consequently, this type of subnet has no traditional network or broadcast address. Packets that would normally be sent to a broadcast address must be handled by other means, such as sending them to the all-hosts multicast address (224.0.0.1).

B. A loopback address is a single host address represented by a /32 prefix, not a /31 prefix which defines a two-address link.
C. A /32 prefix specifies a single host address. A /31 prefix specifies a subnet containing two usable host addresses.
D. A router ID is a 32-bit number, often configured as a /32 loopback address, used to uniquely identify a router within a routing domain, not a /31 link prefix.

1. Internet Engineering Task Force (IETF) RFC 3021: "Using 31-Bit Prefixes on IPv4 Point-to-Point Links", December 2000.

Section 2, "The Solution", Paragraph 1: "For a point-to-point link, it is not necessary to have a network and a broadcast address... A 31-bit subnet mask allows for exactly two host addresses, and does not have a network or broadcast address. This is the most efficient way to number a point-to-point link, as it uses only two addresses." This directly supports the correct answer (A) and refutes the concept of a broadcast address in a /31 subnet.

2. Nokia 7750 SR OS Router Configuration Guide, Release 22.10.R1: "IP Router Configuration > Network Interfaces > IP Address Attributes".

Section: "Assigning an IP Address to a Network Interface": This section details the configuration of IP addresses on Nokia routers. It specifies that a 31-bit prefix length is supported on point-to-point interface types, aligning with the behavior described in RFC 3021. This confirms the technology is relevant to the Nokia certification context.

3. Fall, K. R., & Stevens, W. R. (2011). TCP/IP Illustrated, Volume 1: The Protocols (2nd ed.). Addison-Wesley Professional.

Chapter 3, "IP: Internet Protocol", Section 3.4.2 "Special-Use IPv4 Addresses": While not a direct Nokia source, this foundational academic text discusses special IP addressing cases. It explains that /32 prefixes are used for host-specific routes (like loopbacks) and references RFC 3021 for the use of /31 prefixes on point-to-point links to avoid wasting addresses on network and broadcast identifiers. This supports the reasoning for why options B and C are incorrect.