IP Addresses

Content

IP Addresses
Terms
IPv4 Addresses

Special-Purpose IPv4 Addresses

IPv6 Addresses

Special-Purpose IPv6 Addresses

IP Addresses

Similar to a postal address, the IP address is used to transport data from the sender to the recipient.

An IP address is used for the unique technical addressing of data in a network, such as the Internet or a local area network (LAN). Several IP addresses can be assigned to one computer. Likewise, an IP address can also designate several computers (see broadcast, multicast).

IP addresses in a network are divided into a network part and a host part. The first bits of the binary IP address form the network part and identify the network, such as the address range of an Internet provider, a company network, or your own private LAN. The remaining bits of the binary IP address form the host part and identifies the host (computer, router, printer, etc.) within the network. The separation between network part and host part can be specified e.g. as netmask, as wildcard mask or in the CIDR notation.

Example

For the IP address 198.51.100.0/24, the breakdown is as follows:

This results in the network address 198.51.100.0, the netmask 255.255.255.0 and the broadcast address 198.51.100.255. The addresses 198.51.100.1 - 198.51.100.254 can be assigned to the hosts.

Terms

Network Address

The network address is the first address in a network or subnet. Routers use this address to forward traffic to the appropriate network.

Broadcast Address

The broadcast address allows one computer to send data to all other computers on the same network or subnet simultaneously. The broadcast address is the last address in the subnet. For example, in the network 198.51.100.0/24, the broadcast address is 198.51.100.255.

Multicast

With IP multicast the data is transmitted to several specific recipients simultaneously. This is done via special multicast addresses. These are 224.0.0.0 - 239.255.255.255 for IPv4 and FF00::/8 for IPv6.

Host Addresses

In a network or subnet, the IP addresses between the network address and the broadcast address are available for the hosts. In the network 198.51.100.0/24 these would be the addresses 198.51.100.1 - 198.51.100.254.

Net-Mask

Specifying the netmask or subnet-mask is one way of defining the network part of an IP address. All bits belonging to the network part of the address are set to 1 and all bits in the host part are set to 0.

Wildcard Mask

The wildcard mask is the netmask with inverted bits.

Classless Inter-Domain Routing (CIDR)

The size of a network or subnet can also be specified with the number of bits set to 1 in the network mask instead of with a network mask. This number between 0 and 32 for IPv4 or 128 for IPv6 is given after the IP address, separated by a slash. 198.51.100.0/24 is equivalent to 198.51.100.0/255.255.255.0.

rDNS / in-addr.arpa

The domain in-addr.arpa is a special domain for reverse DNS lookups (rDNS). This can be used to determine the host name for an IP address. For the request, the bytes/octets of the IP address are inserted in reverse order in front of ".in-addr.arpa". Example for IP address 198.51.100.1: 1.100.51.198.in-arr.arpa

IPv4-mapped IPv6 Address

Hybrid dual-stack IPv6/IPv4 implementations (network interfaces that can handle IPv4 and IPv6 data packets) use special IP addresses to map an IPv4 address to an IPv6 address. This is done by prefixing the IPv4 address with a 96-bit prefix consisting of 80 bits of zeros and 16 bits of ones. Example: ::ffff:c633:6401 or also ::ffff:198.51.100.0

IPv6 over IPv4 / 6over4

IPv6 over IPv4 is used to transport IPv6 data packets to IPv4 networks. A link-local IPv6 address is generated from an IPv4 address. For this purpose, the IPv4 address is prefixed with the string "fe80::". Example for IP address 198.51.100.1: fe80::c633:6401

IPv4 Address

IPv4 addresses have a length of 32 bits = 4 bytes, also called octets. These are usually represented as decimal numbers from 0-255 separated by dots. This notation is also called dotted decimal notation.

Example: 198.51.100.25

With IPv4, 2³² or 4,294,967,296 different addresses can be represented.

Special-Purpose IPv4 Addresses

IPv6 Address

IPv6 addresses have a length of 128 bits = 16 bytes. These are represented in hexadecimal notation, with 2 bytes each separated by a colon. Several consecutive blocks with 0 byte each can be abbreviated with two colons. However, only one abbreviation is allowed in one IP.

Example with the same IP in full and abbreviated notations:
2001:db8:000c:0065:0000:0000:0000:01b3
2001:db8:c:65:0:0:0:1b3
2001:db8:c:65::1b3

With IPv6, 2¹²⁸ or 340,282,366,920,938,463,463,374,607,431,768,211,456 different addresses can be represented.

Special-Purpose IPv6 Addresses