Memorize the prefix-to-mask table and you have memorized half of subnetting. This chapter gives you the table, the trick to derive any mask from scratch, and the formulas for hosts-per-subnet and subnets-per-supernet.
/8 255.0.0.0 16,777,214 hosts
/16 255.255.0.0 65,534 hosts
/24 255.255.255.0 254 hosts ← classic LAN
/25 255.255.255.128 126 hosts
/26 255.255.255.192 62 hosts
/27 255.255.255.224 30 hosts
/28 255.255.255.240 14 hosts
/29 255.255.255.248 6 hosts
/30 255.255.255.252 2 hosts ← point-to-point
/31 255.255.255.254 2 hosts ← RFC 3021 PtP
/32 255.255.255.255 1 host ← loopback / routeIf h is the number of host bits, the subnet has 2^h total addresses. Two are reserved: the all-zeros address is the network identifier and the all-ones address is the broadcast. Everything in between is usable for hosts.
RFC 3021 added a special exception: a /31 has only two addresses, but on a strict point-to-point link they can both be used as host addresses. Most modern routers accept /31 today.
1 more section in this chapter, plus a 3-question quiz.
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