An engineer is assigning the address on lo0.0 for a POP router that will serve as the router ID and management anchor for that site. Which subnet mask is the correct choice for that loopback address, and why?
Select an answer to reveal the explanation.
Short Explanation
A loopback isn't a network segment shared with anyone else — it's one router's own address, full stop. A /32 says exactly that: this address means one host and nothing else.
Full Explanation
A /32 mask on lo0.0 correctly expresses that the loopback address identifies a single device rather than a shared broadcast domain or point-to-point link, which is the entire reason a loopback exists as a stable per-router identity. A /24, sized for a subscriber aggregation subnet, wastes address space and implies a shared segment that the loopback does not have — nothing else is meant to sit on that address block. A /30, appropriate for a point-to-point transit circuit between exactly two router interfaces, applies to a physical link with two endpoints, which is not the loopback's role. A /16 compounds the same mistake at a much larger scale, reserving thousands of addresses for a single-host identity and creating confusion about what else might legitimately live in that range. Using /32 consistently across every router's loopback also keeps routing tables and route-filtering policy simple, since each entry unambiguously represents one device. To confirm, show interfaces lo0.0 after commit should display the address with a /32 prefix length, and show route should carry it as a single host route rather than part of a larger advertised block.