IP V4

Internet Protocol version 4 (IPv4) is the fourth revision in the development of the Internet Protocol (IP) and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet. IPv4 is still by far the most widely deployed Internet Layer protocol (As of 2011, IPv6 deployment is still in its infancy).
IPv4 is described in IETF publication RFC 791 (September 1981), replacing an earlier definition (RFC 760, January 1980).
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).

Internet protocol suite
Application layer
DHCP DHCPv6 DNS FTP HTTP IMAP IRC LDAP MGCP NNTP NTP POP RPC RTP RTSP SIP SMTP SNMP SOCKS SSH Telnet TLS/SSL XMPP (more)
Transport layer
TCP UDP DCCP SCTP RSVP RIP BGP ECN (more)
Internet layer
IP IPv4 IPv6 ICMP ICMPv6 IGMP OSPF IPsec (more)
Link layer
ARP/InARP NDP Tunnels L2TP PPP Media access control Ethernet DSL ISDN FDDI (more)
v d e

Contents  [hide]  1 Addressing 1.1 Address representations 1.2 Allocation 1.3 Special-use addresses 1.3.1 Private networks 1.3.1.1 Virtual private networks 1.4 Link-local addressing 1.5 Localhost 1.6 Addresses ending in 0 or 255 1.7 Address resolution 2 Address space exhaustion 3 Packet structure 3.1 Header 3.2 Data 4 Fragmentation and reassembly 4.1 Fragmentation 4.2 Reassembly 5 Assistive protocols 6 See also 7 Notes 8 References 9 External links

[edit] Addressing

IPv4 uses 32-bit (four-byte) addresses, which limits the address space to 4294967296 (2³²) addresses. However, some address blocks are reserved for special purposes such as private networks (~18 million addresses) and multicast addresses (~270 million addresses). This reduces the number of addresses that may be allocated for routing on the public Internet. As addresses are assigned to end users, an IPv4 address shortage has been developing. Network addressing changes by classful network design, Classless Inter-Domain Routing, and network address translation (NAT) have contributed to delay significantly the inevitable exhaustion which occurred on February 3, 2011 when IANA allocated the last five blocks to the five regional Internet registries (RIRs).
This limitation stimulated the development of IPv6 in the 1990s, which has been in commercial deployment since 2006.

[edit] 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 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
Dot-decimal notation	192.0.2.235	N/A
Dotted Hexadecimal[1]	0xC0.0x00.0x02.0xEB	Each octet is individually converted to hexadecimal form
Dotted Octal[1]	0300.0000.0002.0353	Each octet is individually converted into octal
Hexadecimal	0xC00002EB	Concatenation of the octets from the dotted hexadecimal
Decimal	3221226219	The 32-bit number expressed in decimal
Octal	030000001353	The 32-bit number expressed in octal

[edit] Allocation

Originally, an IP address was divided into two parts, the network identifier represented in the most significant (highest order) octet of the address and the host identifier using 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.
Starting around 1985, methods were devised to allow IP networks to be subdivided. The concept of the variable-length subnet mask (VLSM) was introduced which allowed flexible subdivision into varying network sizes.^[2][3]
Around 1993, this system of classes was officially replaced with Classless Inter-Domain Routing (CIDR), and the class-based scheme was dubbed classful, by contrast.
CIDR was designed to permit repartitioning of any address space so that smaller or larger blocks of addresses could be allocated to users. The hierarchical structure created by CIDR is managed by the Internet Assigned Numbers Authority (IANA) and the regional Internet registries (RIRs). Each RIR maintains a publicly-searchable WHOIS database that provides information about IP address assignments.

[edit] Special-use addresses

Reserved address blocks

CIDR address block	Description	Reference
0.0.0.0/8	Current network (only valid as source address)	RFC 1700
10.0.0.0/8	Private network	RFC 1918
127.0.0.0/8	Loopback	RFC 5735
169.254.0.0/16	Link-Local	RFC 3927
172.16.0.0/12	Private network	RFC 1918
192.0.0.0/24	Reserved (IANA)	RFC 5735
192.0.2.0/24	TEST-NET-1, Documentation and example code	RFC 5735
192.88.99.0/24	IPv6 to IPv4 relay	RFC 3068
192.168.0.0/16	Private network	RFC 1918
198.18.0.0/15	Network benchmark tests	RFC 2544
198.51.100.0/24	TEST-NET-2, Documentation and examples	RFC 5737
203.0.113.0/24	TEST-NET-3, Documentation and examples	RFC 5737
224.0.0.0/4	Multicasts (former Class D network)	RFC 3171
240.0.0.0/4	Reserved (former Class E network)	RFC 1700
255.255.255.255	Broadcast	RFC 919

[edit] 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 (RFC 1918):

Name	Address range	Number of addresses	Classful description	Largest CIDR block
24-bit block	10.0.0.0–10.255.255.255	16777216	Single Class A	10.0.0.0/8
20-bit block	172.16.0.0–172.31.255.255	1048576	Contiguous range of 16 Class B blocks	172.16.0.0/12
16-bit block	192.168.0.0–192.168.255.255	65536	Contiguous range of 256 Class C blocks	192.168.0.0/16

[edit] Virtual private networks

Packets with a private destination address are ignored by all public routers. Therefore, it is not possible to communicate directly between two private networks (e.g., two branch offices) via the public Internet. This requires the use of IP tunnels or a virtual private network (VPN).
VPNs establish tunneling connections across the public network such that the endpoints of the tunnel function as routers for packets from the private network. In this routing function the host encapsulates packets in a protocol layer with packet headers acceptable in the public network so that they may be delivered to the opposing tunnel end point where the additional protocol layer is removed and the packet is delivered locally to its intended destination.
Optionally, encapsulated packets may be encrypted to secure the data while it travels over the public network.

[edit] Link-local addressing

RFC 5735 defines an address block, 169.254.0.0/16, for the special use in link-local addressing. These addresses are only valid on the link, such as a local network segment or point-to-point connection, that a host is connected to. These addresses are not routable and like private addresses cannot be the source or destination of packets traversing the Internet. Link-local addresses are primarily used for address autoconfiguration (Zeroconf) 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 mechanisms of address autoconfiguration. Filling the void, Microsoft created an implementation called Automatic Private IP Addressing (APIPA). Due to Microsoft's market power, APIPA has been deployed on millions of machines and has, thus, become a de facto standard in the industry. Many years later, the IETF defined a formal standard for this functionality, RFC 3927, entitled Dynamic Configuration of IPv4 Link-Local Addresses.

[edit] Localhost

The address range 127.0.0.0–127.255.255.255 (127.0.0.0/8 in CIDR notation) is reserved for localhost communication. Addresses within this range should never appear outside a host computer and packets sent to this address are returned as incoming packets on the same virtual network device (known as loopback).

[edit] 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 prefixes /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 case of a /16 subnet 192.168.0.0/255.255.0.0, equivalent to the address range 192.168.0.0–192.168.255.255, the broadcast address is 192.168.255.255. However, one may assign 192.168.1.255, 192.168.2.255, etc. 192.168.0.0 is the network identifier which should not be assigned to an interface,^[4] but 192.168.1.0, 192.168.2.0, etc. may be assigned.
In the past, conflict between network addresses and broadcast addresses arose because some software used non-standard broadcast addresses with zeros instead of ones.^[5]
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.

[edit] 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. DNS is often described in analogy to the telephone system directory information systems in which subscriber names are translated to telephone numbers.

[edit] Address space exhaustion

Since the 1980s it was apparent that the pool of available IPv4 addresses was depleted at a rate that was not initially anticipated in the original design of the network address system.^[6] The apparent threat of exhaustion was the motivation for remedial technologies, such as the introduction of classful networks, the creation of Classless Inter-Domain Routing (CIDR) methods, and network address translation (NAT), and finally for the redesign of the Internet Protocol, based on a larger address format (IPv6).
Several market forces have driven the acceleration of IPv4 address exhaustion:

• Rapidly growing number of Internet users
• Always-on devices — ADSL modems, cable modems
• Mobile devices — laptop computers, PDAs, mobile phones

A variety of technologies introduced during the growth of the Internet have been applied to mitigate IPv4 address exhaustion and its effects, such as:

• Network address translation (NAT) is a technology that masquerades an entire, private network with a single public IP address, permitting the use of private addresses within the private network.
• Use of private networks
• Dynamic Host Configuration Protocol (DHCP)
• Name-based virtual hosting of web sites
• Tighter control by regional Internet registries over the allocation of addresses to local Internet registries
• Network renumbering to reclaim large blocks of address space allocated in the early days of the Internet

The primary address pool of the Internet, maintained by IANA, was exhausted on 3 February 2011 when the last 5 blocks were allocated to the 5 RIRs.^[7][8] APNIC was the first RIR to exhaust its regional pool on 15 April 2011, except for a small amount of address space reserved for the transition to IPv6, which will be allocated under a much more restricted policy.^[9]
The accepted and standardized solution is the migration to Internet Protocol Version 6. The address size was increased in IPv6 to 128 bits, providing a vastly increased address space that also allows improved route aggregation across the Internet and offers large subnetwork allocations of a minimum of 2⁶⁴ host addresses to end-users. Migration to IPv6 is in progress but completion is expected to take considerable time.

[edit] Packet structure

An IP packet consists of a header section and a data section.

[edit] Header

The IPv4 packet header consists of 14 fields, of which 13 are required. The 14th field is optional (red background in table) and aptly named: options. The fields in the header are packed with the most significant byte first (big endian), and for the diagram and discussion, the most significant bits are considered to come first (MSB 0 bit numbering). The most significant bit is numbered 0, so the version field is actually found in the four most significant bits of the first byte, for example.

bit offset	0–3				4–7				8–13						14-15		16–18			19–31
0	Version				Header Length				Differentiated Services Code Point						Explicit Congestion Notification		Total Length
32	Identification																Flags			Fragment Offset
64	Time to Live								Protocol								Header checksum
96	Source IP Address
128	Destination IP Address
160	Options ( if Header Length > 5 )
160 or 192+	Data