Recognize the exact /10 block
02The allocation fixes ten high-order IPv4 bits. It spans second octets 64 through 127, with every possible value of the remaining 22 bits.
With first octet100, compare just-below, first, last and just-above values. Neither every100.x address nor the adjacent /10 belongs to this allocation.
Source for this drawing: RFC 6598 · Read RFC section 7 ↗
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.
100.64.0.0/10 is a specific range, not every address starting with 100.
- §7 allocates 100.64.0.0/10. IPv4 addresses have 32 bits; /10 fixes the first ten and leaves 22 variable bits. The mask is 255.192.0.0 and the block contains 2^22=4,194,304 numeric addresses. This is prefix size, not a promise of that many usable customer hosts on a single subnet.
- The fixed bits are 01100100 01: first octet 100 and the top two bits 01 of the second octet. The second octet’s remaining six bits range from 000000 through 111111, giving decimal 64 through 127.
- The inclusive numeric range is 100.64.0.0 through 100.127.255.255. 100.63.255.255 is just below it and 100.128.0.0 is just above it. The first and last values identify prefix boundaries; this guide does not assign those endpoints to interfaces.
- 100.64.1.2 is inside Shared Address Space. 100.1.2.3 and 100.128.1.2 are outside this /10. Outside this particular block does not automatically mean public, usable, allocated, reachable or safe; other address policies are separate.
- Prefix membership is a routing/address-scope classification, not authentication or a test that a CGN is actually deployed. A diagram marking an address Inside does not grant permission to use it in any network.
The 32-bit strip has ten fixed bits and22 variable bits. The second-octet range picture uses an actual 0–256 scale, with a64-unit selected span. Boundary arithmetic is an offline teaching check.
Read the RFC section ↗