IP and network calculations
Can /31 and /32 networks be useful?
Tools to use
If you learned to subtract two IPv4 addresses for network and broadcast, /31 may appear to have zero usable addresses. Point-to-point links can instead use both. This does not mean ordinary LANs should all become /31. Compare the ranges, identify the usage model, then check actual equipment and topology requirements.
/30 has four addresses and two conventional hosts
192.0.2.0/30
192.0.2.0 … 192.0.2.3
mask: 255.255.255.252Enter 192.0.2.0/30. Two host bits give 2²=4 addresses. In the conventional subnet model, .0 identifies the network and .3 is broadcast, leaving .1 and .2 for endpoints. Four allocated addresses and two host addresses answer different planning questions.
Entering 192.0.2.1/30 yields the same range: the prefix determines its aligned boundary. It does not mean four addresses starting at .1. The next aligned /30 begins at 192.0.2.4.
Both /31 addresses can identify point-to-point ends
192.0.2.0/31
192.0.2.0 … 192.0.2.1
mask: 255.255.255.254/31 has one host bit and two addresses. RFC 3021 uses both as endpoint addresses on point-to-point links, rather than subtracting the first and last as in the conventional model. Read the usage explanation with the number; it is conditional on that link model.
The current output also has a broadcast field showing the range endpoint. For a /31 point-to-point link this does not reserve that address exclusively for broadcast. Read first/last and hostPolicy together instead of applying the generic field label as a rule excluding the endpoint.
/32 identifies one address
192.0.2.10/32
first = last = 192.0.2.10
total = 1/32 has zero host bits and one address. It represents a single host, for example in a host route, not a range containing two endpoints. Subtracting two universally would give nonsense. Separate the shrinking range size from the rules for its use.
A /32 route alone does not prove direct connectivity. Next hop, other routes and interface configuration are separate information. The calculator does not connect to routers or inspect routing tables, so its result is not evidence of reachability.
Check address savings and topology support separately
10 × /30 → 40 addresses
10 × /31 → 20 addressesTen links allocate forty addresses as /30s or twenty as /31s, assuming every link supports the selected model. The arithmetic does not settle support at both ends, management requirements or spare addressing. A smaller count alone is not a deployment plan.
The current VLSM allocator reserves network and broadcast addresses under a conventional IPv4 subnet model. A two-host request producing /30 therefore need not contradict the /31 point-to-point calculation: the models differ. Compare assumptions, not just the shared number two.
Test boundaries without carrying IPv4 subtraction into IPv6
Keep /30, /31 and /32 together as boundary tests. Check endpoints, total and policy, and try another address inside each range. Even /0 has a computable total of 2³²; a mathematical range does not mean every address is usable on one LAN.
IPv6 has no IPv4-style broadcast, so do not carry over the subtraction of two. Use IPv6/CIDR for its 128-bit ranges. Separating address arithmetic from allocation and routing policy makes clear which decisions a calculator can support.