IPV4
IPv4 is a connectionless protocol for use on packet-switched Link Layer networks (e.g., Ethernet). It operates on a best effort delivery model, in that it does not guarantee delivery, nor does it assure proper sequencing or avoidance of duplicate delivery. These aspects, including data integrity, are addressed by an upper layer transport protocol, such as the Transmission Control Protocol (TCP).
Addressing
IPv4 uses 32-bit (four-byte) addresses, which limits the address space to 4294967296 (232) addresses. As addresses were assigned to users, the number of unassigned addresses decreased. IPv4 address exhaustion occurred on February 3, 2011, although it had been significantly delayed by address changes such as classful network design, Classless Inter-Domain Routing, and network address translation (NAT).
This limitation of IPv4 stimulated the development of IPv6 in the 1990s, which has been in commercial deployment since 2006.
IPv4 reserves special address blocks for private networks (~18 million addresses) and multicast addresses (~270 million addresses).
Address representations
IPv4 addresses may be written in any notation expressing a 32-bit integer value, but for human convenience, they are most often written in the dot-decimal notation, which consists of four octets of the address expressed individually in decimal and separated by periods.
The following table shows several representation formats:
Notation Value Conversion from dot-decimal
Dotted decimal 192.0.2.235 N/A
Dotted hexadecimal 0xC0.0x00.0x02.0xEB Each octet is converted to hexadecimal
Dotted octal 0300.0000.0002.0353 Each octet is converted into octal
Hexadecimal 0xC00002EB Concatenation of octet from dotted hexadecimal
Decimal 3221226219 The 32-bit number expressed in decimal
Octal 030000001353 The 32-bit number expressed in octal
Allocation
Originally, an IP address was divided into two parts: the network identifier was the most significant (highest order) octet of the address, and the host identifier was the rest of the address. The latter was therefore also called the rest field. This enabled the creation of a maximum of 256 networks. This was quickly found to be inadequate.
To overcome this limit, the high order octet of the addresses was redefined to create a set of classes of networks, in a system which later became known as classful networking. The system defined five classes, Class A, B, C, D, and E. The Classes A, B, and C had different bit lengths for the new network identification. The rest of an address was used as previously to identify a host within a network, which meant that each network class had a different capacity to address hosts. Class D was allocated for multicast addressing and Class E was reserved for future applications.
Special-use addresses
Reserved address blocks
Range Description
0.0.0.0/8 Current network (only valid as source address)
10.0.0.0/8 Private network
100.64.0.0/10 Shared Address Space
127.0.0.0/8 Loopback
169.254.0.0/16 Link-local
172.16.0.0/12 Private network
192.0.0.0/24 IETF Protocol Assignments
192.0.2.0/24 TEST-NET-1, documentation and examples
192.88.99.0/24 IPv6 to IPv4 relay
192.168.0.0/16 Private network
198.18.0.0/15 Network benchmark tests
198.51.100.0/24 TEST-NET-2, documentation and examples
203.0.113.0/24 TEST-NET-3, documentation and examples
224.0.0.0/4 IP multicast (former Class D network)
240.0.0.0/4 Reserved (former Class E network)
255.255.255.255 Broadcast
Private networks
Of the approximately four billion addresses allowed in IPv4, three ranges of address are reserved for use in private networks. These ranges are not routable outside of private networks, and private machines cannot directly communicate with public networks. They can, however, do so through network address translation.
The following are the three ranges reserved for private networks
Name Address range Classful description Largest CIDR block
24-bit block 10.0.0.0–10.255.255.255 Single Class A 10.0.0.0/8
20-bit block 172.16.0.0–172.31.255.255 Contig. range of 16 Class B 172.16.0.0/12
16-bit block 192.168.0.0–192.168.255.255 Contig. range of 256 Class C 192.168.0.0/16
Virtual private networks
Packets with a private destination address are ignored by all public routers. Two private networks (e.g., two branch offices) cannot communicate via the public internet, unless they use an IP tunnel or a virtual private network (VPN). When one private network wants to send a packet to another private network, the first private network encapsulates the packet in a protocol layer so that the packet can travel through the public network. Then the packet travels through the public network. When the packet reaches the other private network, its protocol layer is removed, and the packet travels to its destination.
Optionally, encapsulated packets may be encrypted to secure the data while it travels over the public network.
Link-local addressing
Defines the special address block 169.254.0.0/16 for link-local addressing. These addresses are only valid on links (such as a local network segment or point-to-point connection) connected to a host. These addresses are not routable. Like private addresses, these addresses cannot be the source or destination of packets traversing the internet. These addresses are primarily used for address autoconfiguration when a host cannot obtain an IP address from a DHCP server or other internal configuration methods.
When the address block was reserved, no standards existed for address autoconfiguration. Microsoft created an implementation called Automatic Private IP Addressing (APIPA), which was deployed on millions of machines and became a de facto standard.
Loopback
The class A network 127.0.0.0 (classless network 127.0.0.0/8) is reserved for loopback. IP packets which source addresses belong to this network should never appear outside a host. The modus operandi of this network expands upon that of a loopback interface:
- IP packets which source and destination addresses belong to the network (or subnetwork) of the same loopback interface are returned back to that interface;
- IP packets which source and destination addresses belong to networks (or subnetworks) of different interfaces of the same host, one of them being a loopback interface, are forwarded regularly.
Addresses ending in 0 or 255
Networks with subnet masks of at least 24 bits, i.e. Class C networks in classful networking, and networks with CIDR suffixes /24 to /32 (255.255.255.0–255.255.255.255) may not have an address ending in 0 or 255.
Classful addressing prescribed only three possible subnet masks: Class A, 255.0.0.0 or /8; Class B, 255.255.0.0 or /16; and Class C, 255.255.255.0 or /24. For example, in the subnet 192.168.5.0/255.255.255.0 (192.168.5.0/24) the identifier 192.168.5.0 commonly is used to refer to the entire subnet. To avoid ambiguity in representation, the address ending in the octet 0 is reserved.
A broadcast address is an address that allows information to be sent to all interfaces in a given subnet, rather than a specific machine. Generally, the broadcast address is found by obtaining the bit complement of the subnet mask and performing a bitwise OR operation with the network identifier. In other words, the broadcast address is the last address in the address range of the subnet. For example, the broadcast address for the network 192.168.5.0 is 192.168.5.255. For networks of size /24 or larger, the broadcast address always ends in 255.
However, this does not mean that every address ending in 0 or 255 cannot be used as a host address. For example, in the /16 subnet 192.168.0.0/255.255.0.0, which is equivalent to the address range 192.168.0.0–192.168.255.255, the broadcast address is 192.168.255.255. One can use the following addresses for hosts, even though they end with 255: 192.168.1.255, 192.168.2.255, etc. Also, 192.168.0.0 is the network identifier and must not be assigned to an interface. The addresses 192.168.1.0, 192.168.2.0, etc., may be assigned, despite ending with 0.
In the past, conflict between network addresses and broadcast addresses arose because some software used non-standard broadcast addresses with zeros instead of ones.
In networks smaller than /24, broadcast addresses do not necessarily end with 255. For example, a CIDR subnet 203.0.113.16/28 has the broadcast address 203.0.113.31.
Address resolution
Hosts on the Internet are usually known by names, e.g., www.example.com, not primarily by their IP address, which is used for routing and network interface identification. The use of domain names requires translating, called resolving, them to addresses and vice versa. This is analogous to looking up a phone number in a phone book using the recipient's name.
The translation between addresses and domain names is performed by the Domain Name System (DNS), a hierarchical, distributed naming system which allows for subdelegation of name spaces to other DNS servers.
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