Read the three reserved prefixes
02A slash prefix fixes the leading bits of an IPv4 address. The remaining bits vary within the block; a 12-bit prefix can split an eight-bit octet.
172.15.255.255 and 172.32.0.0 are outside RFC 1918; 172.16.0.0 and 172.31.255.255 are inside. Outside does not mean public or assigned.
Source for this drawing: RFC 1918 · Read RFC section 3 ↗
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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.
172.16/12 includes second octets 16 through 31—not all of 172/8.
- The reserved blocks are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Their inclusive ranges are exactly those listed in §3.
- They contain 2^24, 2^20 and 2^16 address values respectively. These are block sizes, not counts of usable host addresses on every possible subnet.
- The original “24-bit block”, “20-bit block” and “16-bit block” names count variable bits; modern slash notation counts fixed prefix bits. Addresses outside these three blocks are not thereby guaranteed globally routable or free to use.
A chosen 172.20.1.9 example has bits 10101100 00010100 00000001 00001001. Its first twelve bits are 10101100 0001, matching 172.16/12. The prefix bar is proportional to bit counts; the range table is not a map of physical networks. Do not assign a displayed block endpoint to a host without applying that subnet’s own rules.
Read the RFC section ↗