HON95-wiki/network/ipv4.md

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Special Prefixes

|Prefix|Description| |-|-| | 0.0.0.0/8 | Current network | | 10.0.0.0/8 | Private network | | 100.64.0.0/10 | Shared address space for CGN | | 127.0.0.0/8 | Localhost | | 169.254.0.0/16 | Link-local autoconfiguration | | 172.16.0.0/12 | Private network | | 192.0.0.0/24 | IETF Protocol Assignments | | 192.0.2.0/24 | Documentation (TEST-NET-1) | | 192.18.0.0/15 | Inter-network benchmarking | | 192.51.100.0/24 | Documentation (TEST-NET-2) | | 192.88.99.0/24 | 6to4 anycast (deprecated) | | 192.168.0.0/16 | Private network | | 203.0.113.0/24 | Documentation (TEST-NET-3) | | 224.0.0.0/4 | Multicast (formerly Class D) | | 240.0.0.0/4 | Reserved (formerly class E) | | 255.255.255.255/32 | Limited broadcast |

Addressing

Classful Routing

Originally the IPv4 address space was split into five classes with fixed, implicit subnet masks, as seen below:

| Class | Leading bits | First address | Network bits | Purpose | | - | - | - | | A | 0 | 0.0.0.0 | 8 | Unicast | | B | 10| 128.0.0.0 | 16 | Unicast | | C | 110 | 192.0.0.0 | 24 | Unicast | | D | 1110 | 224.0.0.0 | N/A | Multicast | | E | 1111 | 240.0.0.0 | N/A | Reserved |

VLSM and CIDR

Variable-length subnet masking (VLSM) allows splitting networks into multiple smaller networks (subnetting). It is the opposite of fixed-length subnet masking.

Classless inter-domain routing (CIDR) allows combining multiple smaller networks (with a common prefix) into a larger network (supernetting). It is the opposite of classful routing.

The terms are frequently interchanged and now typically used to refer to the same thing.

Special Addresses

• The first address in a network is reserved for identifying the network and cannot be used by any hosts.
• The last address in the network is reserved for directed broadcasts targeted at all hosts within the certain network. It it routable and frequently blocked.

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