Spend two addresses on two endpoints
01A point-to-point link has exactly two ends. A /30 reserves four IPv4 address values; a /31 can number the same two endpoints with two values.
500 × 4 = 2000 versus 500 × 2 = 1000 address values. Two endpoint identities remain on every link.
Source for this drawing: RFC 3021 · Read RFC section 1 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
Conserve address space without leaving either endpoint unnumbered.
- A numbered /30 commonly uses two endpoint addresses plus the all-zero network value and all-one directed-broadcast value. RFC 3021 changes this allocation only for point-to-point links.
- A /31 reserves two address values, and both identify endpoints. Compared with a /30, the allocation per link is halved; it does not double bandwidth.
- For 500 links, /30 allocations use 2000 values and /31 allocations use 1000, saving 1000. This arithmetic is the RFC’s example, not a measured production network.
All 192.0.2.x addresses are fictional documentation values from RFC 5737, not addresses to deploy. The diagrams compare separate plans; they do not renumber a live link automatically. Unnumbered links and PPP host-address alternatives are mentioned in §1 but are not specified here.
Read the RFC section ↗