IPv4 Subnetting Reference

Certain chunks of IPv4 address space are specially allocated by RFCs for special uses such as loopback (RFC 1643), Private networks (RFC 1918), and Zeroconf (RFC 3927) usage, and are not available for allocation by Regional Internet Registries (RIRs).

The netmask is a bitmask that can be used to separate the bits of the network identifier from the bits of the host identifier. It is often written in the same notation used to denote IP addresses.
Not all sizes of prefix announcement may be routable on the public Internet: see routing, peering.

 

Class	Leading bits	Start	End	Default Subnet Mask in dotted decimal	CIDR notation
A	0	0.0.0.0	126.255.255.255	255.0.0.0	/8
B	10	128.0.0.0	192.255.255.255	255.255.0.0	/16
C	110	192.0.0.0	223.255.255.255	255.255.255.0	/24
D	1110	224.0.0.0	239.255.255.255	not defined	not defined
E	1111	240.0.0.0	255.255.255.0	not defined	not defined

The 127.0.0.1 network is left out because it is designated for loopback and cannot be assigned to a network
Class D multicasting
Class E reserved

CIDR	Netmask	Host / Subnet	Classful Name	Typical Usage
/8	255.0.0.0	16777216	Class A	Largest block allocation made by IANA
/9	255.128.0.0	8388608
/10	255.192.0.0	4194304
/11	255.224.0.0	2097152
/12	255.240.0.0	1048576
/13	255.248.0.0	524288
/14	255.252.0.0	262144
/15	255.254.0.0	131072
/16	255.255.0.0	65536	Class B
/17	255.255.128.0	32768		ISP / large business
/18	255.255.192.0	16384		ISP / large business
/19	255.255.224.0	8192		ISP / large business
/20	255.255.240.0	4096		Small ISP / large business
/21	255.255.248.0	2048		Small ISP / large business
/22	255.255.252.0	1024
/23	255.255.254.0	512
/24	255.255.255.0	256	Class C	Large LAN
/25	255.255.255.128	128		Large LAN
/26	255.255.255.192	64		Large LAN
/27	255.255.255.224	32		Large LAN
/28	255.255.255.240	16		Large LAN
/29	255.255.255.248	8
/30	255.255.255.252	4		"Glue network" (point to point links)
/31	255.255.255.254	2		"Useless Network", proposed for point to point links (RFC 3021)
/32	255.255.255.255	1		Host route

Note that in common usage, the "host all zeros" address is reserved for referring to the whole network, while the "host all ones" address is reserved as a broadcast address; this reduces the number of hosts available by 2, explaining the reference to /31 as "Useless Network".

 

CIDR

Classless Inter-Domain Routing (CIDR, pronounced "cider") was introduced in 1993 and is the latest refinement to the way IP addresses are interpreted. It replaced the previous generation of IP address syntax, classful networks. Specifically, rather than allocating address blocks on eight-bit (i.e., octet) boundaries forcing 8, 16, or 24-bit prefixes, it used the technique of variable-length subnet masking (VLSM) to allow allocation on arbitrary-length prefixes. CIDR encompasses:

• The VLSM technique of specifying arbitrary length prefix boundaries. A CIDR-compliant address is written with a suffix indicating the number of bits in the prefix length, such as 192.168.0.0/16. This permits more efficient use of increasingly scarce IPv4 addresses.
• The aggregation of multiple contiguous prefixes into supernets, and, wherever possible in the Internet, advertising aggregates, thus reducing the number of entries in the global routing table. Aggregation hides multiple levels of subnetting from the Internet routing table, and reverses the process of "subnetting a subnet" with VLSM.
• The administrative process of allocating address blocks to organizations based on their actual and short-term projected need, rather than the very large or very small blocks required by classful addressing schemes.

IPv6 utilizes the CIDR convention of indicating prefix length with a suffix, but the longer address field of IPv6 made it unnecessary to practice great economy in allocating the minimum amount of address space an organization could justify. The concept of class was never used in IPv6.

Taken from : From Wikipedia, the free encyclopedia

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