IP Address

Internal

• Linux Network Concepts
• IP

Overview

IP addresses allow network resources to be reached through a network interface. Each IP address must be unique in its own network. Networks can be isolated from each other, or they can be bridged or translated (see NAT).

IPv4 Address

An IPv4 address consists in 32 bits (4 bytes). Each byte is divided by a period and typically expressed as a number 0-255. Example

192.168.1.1

IPv4 Address Classes

   Bit -->  0                           31            Address Range:
           +-+----------------------------+
           |0|       Class A Address      |       0.0.0.0 - 127.255.255.255
           +-+----------------------------+
           +-+-+--------------------------+
           |1 0|     Class B Address      |     128.0.0.0 - 191.255.255.255
           +-+-+--------------------------+
           +-+-+-+------------------------+
           |1 1 0|   Class C Address      |     192.0.0.0 - 223.255.255.255
           +-+-+-+------------------------+
           +-+-+-+-+----------------------+
           |1 1 1 0|  MULTICAST Address   |     224.0.0.0 - 239.255.255.255
           +-+-+-+-+----------------------+
           +-+-+-+-+-+--------------------+
           |1 1 1 1 0|     Reserved       |     240.0.0.0 - 247.255.255.255
           +-+-+-+-+-+--------------------+

Each of the address classes have a range within them that is used to designate private (not routed) networks. Any computer that is not connect to internet directly can use these addresses.

Class A

0xxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

If the first bit of the IPv4 address is 0, in means that the address is part of class A. The address falls within the range 0.0.0.0 - 127.255.255.255.

Loopback Range

The loopback range is specified by addresses from 127.0.0.0 to 127.255.255.255. Addresses in this range are used by each host to test networking to itself. Typically, 127.0.0.1 is used.

Private Network Assignment within Class A

10.0.0.0-10.255.255.255

Class B

10xxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

If the first two bits of the address are 10, the address is in class B. The address falls within the range 128.0.0.0 - 191.255.255.255.

Private Network Assignment within Class B

172.16.0.0-172.31.255.255

Example 170.20.1.1/255.255.0.0 (/16)

Class C

110xxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

If the first three bits of the address are 110, the address is in class C. The address falls within the range 192.0.0.0 - 223.255.255.255.

Private Network Assignment within Class C

192.168.0.0-192.168.255.255

Class D

1110xxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

If the first four bits of the address are 1110, the address is in class D. The address falls within the range 224.0.0.0 - 239.255.255.255.

Class D is reserved fro multicast protocols.

Class E

1111xxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

If the first four bits of the address are 1111, the address is in class E. The address falls within the range 224.0.0.0 - 255.255.255.255.

Class E addresses are reserved for future and experimental use, and are largely not used.

Network Address and Host Address

IPv4 addresses are a concatenation of two separate components. The first part of the address is used to identify the network the address is part of. The second part, that comes afterwards, designates a specific host within that network. Note that the host bits can be further reallocated to represent subnets within a network, effectively enlarging the network address space on the expense of host address space. See subnets below.

Class A addresses use the remainder of the first octet (7 bits) to represent the network and the rest of the three octets to define hosts.

Class B addresses use the first two octets (6 + 8 = 14 bits) to represent the network and the rest of the two octets to define hosts.

Class C addresses use the first three octets (5 + 8 + 8 = 21bits) to represent the network and the rest of 1 octet to define hosts.

Within the host address space, 0 and 11...11 are reserved and cannot used as host addresses.

For example, for a C class address, 0 and 256 cannot be used as host addresses.

Network Address, Netmasks, Subnets and Subnet Maks

Each IP address class defines what part of the IP address is used to designate the network.

Netmask

A net mask defines what bits of the IP address are used to designate the network. Traditionally, that is represented by turning those bits to 1, which makes the netmask a 32 bit number filled with all-1s in the network part of the IP address, and with all-0s in the host part.:

Class	Default Netmask	Default Netmask (different notation)
A	11111111.00000000.00000000.00000000	255.0.0.0	/8
B	11111111.11111111.00000000.00000000}}	255.255.0.0	/16
C	11111111.11111111.11111111.00000000}}	255.255.255.0	/24

We determine the network portion of the address by applying a bitwise AND operation to between the address and the net mask.

The netmask is also known as network prefix.

Example

For class C address 192.168.0.1, the first three bytes are used to address the network (192.168.0.0) and the last byte is used to address the hosts within the network. 1 is an address out of 254 unique addresses allowed by the space (the address space allows for 256 unique values, but 0 and 256 cannot be used, as they are reserved). The netmaks in this case is 11111111.11111111.11111111.00000000 or 255.255.255.0.

Subnets

By default, each network has one subnet, which contains all hosts that can be defined in that network. However, some of the bits used to represent the host address can be reallocated as additional network specification, thus allowing for the definition of additional subnets within the network.

Do reallocated bits need to be adjacent to the network portion?

If subnetting is used, the netmask becomes the subnet mask. A subnet masks is essentially a netmask with additional bits reallocated from host address representation to network address representation.

Example

In the above example we might decide we don't need a host address space that allows 256 hosts, so we re-allocate 1 bit to the network address space. The netmask becomes the subnet mask and it is 11111111.11111111.11111111.10000000 or 255.255.255.128.

Traditionally, the subnet itself must not be used as an address.

The CIDR (Classless Inter-Domain Routing) Notation

RFC4632

The CIDR notation is a specification added to the IP address itself as to the number of significant bits that make up the routing or networking portion:

X.X.X.X/<the-number-of-first-bits-of-the-ip-address-that-represent-the-network-address>

The CIDR number comes from the number of 1's in the subnet mask when converted to binary.

Example

In the above subnetting example, the CIDR notation is 192.168.0.1/25.

CIDR Values

Subnet Mask	CIDR Prefix	Total IP's	Usable IP's	Number of Class C networks
255.255.255.255	/32	1	1	1/256th
255.255.255.254	/31	2	0	1/128th
255.255.255.252	/30	4	2	1/64th
255.255.255.248	/29	8	6	1/32nd
255.255.255.240	/28	16	14	1/16th
255.255.255.224	/27	32	30	1/8th
255.255.255.192	/26	64	62	1/4th
255.255.255.128	/25	128	126	1 half
255.255.255.0	/24	256	254	1
255.255.254.0	/23	512	510	2
255.255.252.0	/22	1024	1022	4
255.255.248.0	/21	2048	2046	8
255.255.240.0	/20	4096	4094	16
255.255.224.0	/19	8192	8190	32
255.255.192.0	/18	16,384	16,382	64
255.255.128.0	/17	32,768	32,766	128
255.255.0.0	/16	65,536	65,534	256
255.254.0.0	/15	131,072	131,070	512
255.252.0.0	/14	262,144	262,142	1024
255.248.0.0	/13	524,288	524,286	2048
255.240.0.0	/12	1,048,576	1,048,574	4096
255.224.0.0	/11	2,097,152	2,097,150	8192
255.192.0.0	/10	4,194,304	4,194,302	16,384
255.128.0.0	/9	8,388,608	8,388,606	32,768
255.0.0.0	/8	16,777,216	16,777,214	65,536
254.0.0.0	/7	33,554,432	33,554,430	131,072
252.0.0.0	/6	67,108,864	67,108,862	262,144
248.0.0.0	/5	134,217,728	134,217,726	1,048,576
240.0.0.0	/4	268,435,456	268,435,454	2,097,152
224.0.0.0	/3	536,870,912	536,870,910	4,194,304
192.0.0.0	/2	1,073,741,824	1,073,741,822	8,388,608
128.0.0.0	/1	2,147,483,648	2,147,483,646	16,777,216
0.0.0.0	/0	4,294,967,296	4,294,967,294	33,554,432

In Range

A subnet 10.0.1.0/24 is not within range of the 10.7.0.0/16 subnet, whereas 10.7.1.0/24 is.

IPv6 Address

An IPv6 address consists in 128 bits (16 bytes). An IPv6 address is generally written out as eight segments of four hexadecimal digits:

1203:8fe0:fe80:b897:8990:8a7c:99bf:323d

Typical IPv6 netmasks are be 128 (::1/128) or 10 (fe80::203:baff:fe27:1243/10).