IPv4

Internet Protocol version 4 (IPv4) is the fourth revision in the development of the Internet Protocol (IP) and it is 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 2010, 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 (e.g., Transmission Control Protocol).

Addressing

IPv4 uses 32-bit (four-byte) addresses, which limits the address space to 4,294,967,296 (2³²) possible unique addresses. However, some are reserved for special purposes such as private networks (~18 million addresses) or multicast addresses (~270 million addresses). This reduces the number of addresses that can potentially be allocated for routing on the public Internet. As addresses are being incrementally delegated to end users, an IPv4 address shortage has been developing. However, network addressing architecture redesign via classful network design, Classless Inter-Domain Routing, and network address translation (NAT) has contributed to delay significantly the inevitable exhaustion.

This limitation has stimulated the development of IPv6, which is currently in the early stages of deployment, and is the only long-term solution.

Address representations

IPv4 addresses may simply 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 the four octets of the address expressed separately 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	0xC0.0x00.0x02.0xEB	Each octet is individually converted to hexadecimal form
Dotted Octal	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

Additionally, in dotted format, each octet can be of any of the different bases. For example, 192.0x00.0002.235 is a valid (though unconventional) equivalent to the above addresses.

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. Quickly this was 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.

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.

Special-use addresses

Main article: Reserved IP addresses § Reserved IPv4 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

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

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.

Link-local addressing

Main article: Link-local address

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.

Localhost

Main article: 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).

Addresses ending in 0 or 255

Main article: IPv4 subnetting reference

It is a common misunderstanding that addresses ending with an octet of 0 or 255 can never be assigned to hosts. (See also Subnet - all zeros or all ones.) This is only true of networks with subnet masks of at least 24 bits — Class C networks in the old classful addressing scheme, or in CIDR, networks with masks of /24 to /32 (or 255.255.255.0–255.255.255.255).

In classful addressing (now obsolete with the advent of CIDR), there are 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 (or 192.168.5.0/24) the identifier 192.168.5.0 refers to the entire subnet, so it cannot also refer to an individual device in that subnet.

A broadcast address is an address that allows information to be sent to all machines on 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 range belonging to the subnet. In our example, the broadcast address would be 192.168.5.255, so to avoid confusion this address also cannot be assigned to a host. On a Class A, B, or C subnet, the broadcast address always ends in 255.

However, this does not mean that every addresses ending in 255 cannot be used as a host address. For example, in the case of a Class B subnet 192.168.0.0/255.255.0.0 (or 192.168.0.0/16), equivalent to the address range 192.168.0.0–192.168.255.255, the broadcast address is 192.168.255.255. However, one can assign 192.168.1.255, 192.168.2.255, etc. (though this can cause confusion). Also, 192.168.0.0 is the network identifier and so cannot be assigned, but 192.168.1.0, 192.168.2.0, etc. can be assigned (though this can also cause confusion).

With the advent of CIDR, broadcast addresses do not necessarily end with 255.

In general, the first and last addresses in a subnet are used as the network identifier and broadcast address, respectively. All other addresses in the subnet can be assigned to hosts on that subnet.

Address resolution

Main article: Domain Name System

Hosts on the Internet are usually known not by IP addresses, but by names (e.g., en.wikipedia.org, www.whitehouse.gov, www.freebsd.org, www.berkeley.edu). The routing of IP packets across the Internet is not directed by such names, but by the numeric IP addresses assigned to such domain names. This requires translating (or resolving) domain names to addresses.

The Domain Name System (DNS) provides such a system for converting names to addresses and addresses to names. Much like CIDR addressing, the DNS naming is also hierarchical and allows for subdelegation of name spaces to other DNS servers.

The domain name system is often described in analogy to the telephone system directory information systems in which subscriber names are translated to telephone numbers.

Address space exhaustion

Main article: IPv4 address exhaustion

Since the 1980s it has been apparent that the number of available IPv4 addresses is being exhausted at a rate that was not initially anticipated in the design of the network. This was the motivation for the introduction of classful networks, for the creation of CIDR addressing, and finally for the redesign of the Internet Protocol, based on a larger address format (IPv6).

Today, there are several driving forces for 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

The accepted and standardized solution is the migration to IPv6. The address size in IPv6 was increased from 32 bits in IPv4 to 128 bits, providing a vastly increased address space that 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 is expected to take considerable time.

Methods to mitigate the IPv4 address exhaustion are:

• Network address translation (NAT)
• Use of private networks
• Dynamic Host Configuration Protocol (DHCP)
• Name-based virtual hosting
• Tighter control by Regional Internet Registries on 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

As of October 2010 predictions of exhaustion date of the unallocated IANA pool converge to between January 2011 and January 2012

Network address translation

Main article: Network address translation

The rapid pace of allocation of the IPv4 addresses and the resulting shortage of address space since the early 1990s led to several methods of more efficient use. One method was the introduction of network address translation (NAT). NAT devices masquerade an entire, private network 'behind' a single public IP address, permitting the use of private addresses within the private network. Most mass-market consumer Internet access providers rely on this technique.

Packet structure

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

Header

The IPv4 packet header consists of 13 fields, of which 12 are required. The 13^th 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

Version

The first header field in an IP packet is the four-bit version field. For IPv4, this has a value of 4 (hence the name IPv4).

Internet Header Length (IHL)

The second field (4 bits) is the Internet Header Length (IHL) telling the number of 32-bit words in the header. Since an IPv4 header may contain a variable number of options, this field specifies the size of the header (this also coincides with the offset to the data). The minimum value for this field is 5 (RFC 791), which is a length of 5×32 = 160 bits = 20 bytes. Being a 4-bit value, the maximum length is 15 words (15×32 bits) or 480 bits = 60 bytes.

Differentiated Services Code Point (DSCP)

Originally defined as the Type of Service field, this field is now defined by RFC 2474 for Differentiated services (DiffServ). New technologies are emerging that require real-time data streaming and therefore will make use of the DSCP field. An example is Voice over IP (VoIP) that is used for interactive data voice exchange.

Explicit Congestion Notification (ECN)

Defined in RFC 3168 and allows end-to-end notification of network congestion without dropping packets. ECN is an optional feature that is only used when both endpoints support it and are willing to use it. It is only effective when supported by the underlying network.

Total Length

This 16-bit field defines the entire datagram size, including header and data, in bytes. The minimum-length datagram is 20 bytes (20-byte header + 0 bytes data) and the maximum is 65,535 — the maximum value of a 16-bit word. The minimum size datagram that any host is required to be able to handle is 576 bytes, but most modern hosts handle much larger packets. Sometimes subnetworks impose further restrictions on the size, in which case datagrams must be fragmented. Fragmentation is handled in either the host or packet switch in IPv4 (see Fragmentation and reassembly).

Identification

This field is an identification field and is primarily used for uniquely identifying fragments of an original IP datagram. Some experimental work has suggested using the ID field for other purposes, such as for adding packet-tracing information to datagrams in order to help trace back datagrams with spoofed source addresses.

Flags

A three-bit field follows and is used to control or identify fragments. They are (in order, from high order to low order):

• bit 0: Reserved; must be zero.
• bit 1: Don't Fragment (DF)
• bit 2: More Fragments (MF)

If the DF flag is set and fragmentation is required to route the packet then the packet will be dropped. This can be used when sending packets to a host that does not have sufficient resources to handle fragmentation.

When a packet is fragmented all fragments have the MF flag set except the last fragment, which does not have the MF flag set. The MF flag is also not set on packets that are not fragmented — an unfragmented packet is its own last fragment.

Fragment Offset

The fragment offset field, measured in units of eight-byte blocks, is 13 bits long and specifies the offset of a particular fragment relative to the beginning of the original unfragmented IP datagram. The first fragment has an offset of zero. This allows a maximum offset of (2¹³ – 1) × 8 = 65,528 bytes which would exceed the maximum IP packet length of 65,535 bytes with the header length included (65,528 + 20 = 65,548 bytes).

Time To Live (TTL)

An eight-bit time to live field helps prevent datagrams from persisting (e.g. going in circles) on an internet. This field limits a datagram's lifetime. It is specified in seconds, but time intervals less than 1 second are rounded up to 1. In latencies typical in practice, it has come to be a hop count field. Each router that a datagram crosses decrements the TTL field by one. When the TTL field hits zero, the packet is no longer forwarded by a packet switch and is discarded. Typically, an ICMP message (specifically the time exceeded) is sent back to the sender that it has been discarded. The reception of these ICMP messages is at the heart of how traceroute works.

Protocol

This field defines the protocol used in the data portion of the IP datagram. The Internet Assigned Numbers Authority maintains a list of IP protocol numbers which was originally defined in RFC 790.

Header Checksum

The 16-bit checksum field is used for error-checking of the header. At each hop, the checksum of the header must be compared to the value of this field. If a header checksum is found to be mismatched, then the packet is discarded. Note that errors in the data field are up to the encapsulated protocol to handle — indeed, both UDP and TCP have checksum fields.

Since the TTL field is decremented on each hop and fragmentation is possible at each hop then at each hop the checksum will have to be recomputed. The method used to compute the checksum is defined within RFC 1071:

The checksum field is the 16-bit one's complement of the one's complement sum of all 16-bit words in the header. For purposes of computing the checksum, the value of the checksum field is zero.

In other words, all 16-bit words are summed together using one's complement (with the checksum field set to zero). The sum is then one's complemented and this final value is inserted as the checksum field.

For example, use Hex 45000030442240008006442e8c7c19acae241e2b (20 bytes IP header):

4500 + 0030 + 4422 + 4000 + 8006 + 0000 + 8c7c + 19ac + ae24 + 1e2b = 2BBCF

2 + BBCF = BBD1 = 1011101111010001, the 1'S of sum = 0100010000101110 = 442E

To validate a header's checksum the same algorithm may be used - the checksum of the header with the checksum field filled in should be a word containing all zeros (value 0).

Source address

An IPv4 address is a group of four octets for a total of 32 bits. The value for this field is determined by taking the binary value of each octet and concatenating them together to make a single 32-bit value.

For example, the address 10.9.8.7 would be 00001010000010010000100000000111.

This address is the address of the sender of the packet. Note that this address may not be the "true" sender of the packet due to network address translation. Instead, the source address will be translated by the NATing machine to its own address. Thus, reply packets sent by the receiver are routed to the NATing machine, which translates the destination address to the original sender's address.

Destination address

Identical to the source address field but indicates the receiver of the packet.

Options

Additional header fields may follow the destination address field, but these are not often used. Note that the value in the IHL field must include enough extra 32-bit words to hold all the options (plus any padding needed to ensure that the header contains an integral number of 32-bit words). The list of options may be terminated with an EOL (End of Options List, 0x00) option; this is only necessary if the end of the options would not otherwise coincide with the end of the header. The possible options that can be put in the header are as follows:

Field	Size (bits)	Description
Copied	1	Set to 1 if the options need to be copied into all fragments of a fragmented packet.
Option Class	2	A general options category. 0 is for "control" options, and 2 is for "debugging and measurement". 1, and 3 are reserved.
Option Number	5	Specifies an option.
Option Length	8	Indicates the size of the entire option (including this field). This field may not exist for simple options.
Option Data	Variable	Option-specific data. This field may not exist for simple options.

• Note: If the Header Length is greater than 5, i.e. it is between 6-15, it means that the Options field is present and must be considered.
• Note: the Copied, Option Class, and Option Number are sometimes referred to as a single eight-bit field - the Option Type.

The use of the LSRR and SSRR options (Loose and Strict Source and Record Route) is discouraged because they create security concerns; many routers block packets containing these options.

Data

The last field is not a part of the header and, consequently, not included in the checksum field. The contents of the data field are specified in the protocol header field and can be any one of the transport layer protocols.

Some of the most commonly used protocols are listed below including their value used in the protocol field:

• 1: Internet Control Message Protocol (ICMP)
• 2: Internet Group Management Protocol (IGMP)
• 6: Transmission Control Protocol (TCP)
• 17: User Datagram Protocol (UDP)
• 41: IPv6 encapsulation
• 89: Open Shortest Path First (OSPF)
• 132: Stream Control Transmission Protocol (SCTP)

See List of IP protocol numbers for a complete list.

Fragmentation and reassembly

Main article: IP fragmentation

The Internet Protocol is the facility in the Internet architecture that enables different networks to exchange traffic and route traffic across one another. The design accommodates networks of diverse physical nature; it is independent of the underlying transmission technology used in the Link Layer. Link Layer networks of different hardware design usually vary not only in transmission speed, but also in the structure and size of valid framing methods, characterized by the maximum transmission unit (MTU) parameter. To fulfill the role of IP to traverse networks, it was necessary to implement a mechanism to automatically adjust the size of transmission units to adapt to the underlying technology. This introduced the need for fragmentation of IP datagrams. In IPv4, this function was placed at the Internet Layer, and is performed in IPv4 routers, which thus only require this layer as highest one implemented in their design.

In contrast, the next generation of the Internet Protocol, namely IPv6, does not require routers to perform fragmentation; instead, hosts must determine the path maximum transmission unit in advance of transmission and send conforming datagrams.

Fragmentation

When a device receives an IP packet it examines the destination address and determines the outgoing interface to use. This interface has an associated MTU that dictates the maximum data size for its payload. If the data size is bigger than the MTU then the device must fragment the data.

The device then segments the data into segments where each segment is less-than-or-equal-to the MTU less the IP header size (20 bytes minimum; 60 bytes maximum). Each segment is then put into its own IP packet with the following changes:

• The total length field is adjusted to the segment size
• The more fragments (MF) flag is set for all segments except the last one, which is set to 0
• The fragment offset field is set accordingly based on the offset of the segment in the original data payload. This is measured in units of eight-byte blocks.
• The header checksum field is recomputed.

For example, for an IP header of length 20 bytes and an Ethernet MTU of 1,500 bytes the fragment offsets would be: 0, (1480/8) = 185, (2960/8) = 370, (4440/8) = 555, (5920/8) = 740, etc.

By some chance if a packet changes link layer protocols or the MTU reduces then these fragments would be fragmented again.

For example, if a 4,500-byte data payload is inserted into an IP packet with no options (thus total length is 4,520 bytes) and is transmitted over a link with an MTU of 2,500 bytes then it will be broken up into two fragments:

#	Total length		More fragments (MF) flag set?	Fragment offset
	Header	Data
1	2500		Yes	0
	20	2480
2	2040		No	310
	20	2020

Now, let's say the MTU drops to 1,500 bytes. Each fragment will individually be split up into two more fragments each:

#	Total length		More fragments (MF) flag set?	Fragment offset
	Header	Data
1	1500		Yes	0
	20	1480
2	1020		Yes	185
	20	1000
3	1500		Yes	310
	20	1480
4	560		No	495
	20	540

Indeed, the amount of data has been preserved — 1480 + 1000 + 1480 + 540 = 4500 — and the last fragment offset (495) * 8 (bytes) plus data — 3960 + 540 = 4500 — is also the total length.

Note that fragments 3 & 4 were derived from the original fragment 2. When a device must fragment the last fragment then it must set the flag for all but the last fragment it creates (fragment 4 in this case). Last fragment would be set to 0 value.

Reassembly

When a receiver detects an IP packet where either of the following is true:

• "more fragments" flag set
• "fragment offset" field is non-zero

then the receiver knows the packet is a fragment. The receiver then stores the data with the identification field, fragment offset, and the more fragments flag. When the receiver receives a fragment with the more fragments flag set to 0 then it knows the length of the original data payload since the fragment offset multiplied by 8 (bytes) plus the data length is equivalent to the original data payload size.

Using the example above, when the receiver receives fragment 4 the fragment offset (495 or 3960 bytes) and the data length (540 bytes) added together yield 4500 — the original data length.

Once it has all the fragments then it can reassemble the data in proper order (by using the fragment offsets) and pass it up the stack for further processing.

Assistive protocols

The Internet Protocol is the protocol that defines and enables internetworking at the Internet Layer and thus forms the Internet. It uses a logical addressing system. IP addresses are not tied in any permanent manner to hardware identifications and, indeed, a network interface can have multiple IP addresses. Hosts and routers need additional mechanisms to identify the relationship between device interfaces and IP addresses, in order to properly deliver an IP packet to the destination host on a link. The Address Resolution Protocol (ARP) perform this IP address to hardware address (MAC address) translation for IPv4. In addition the reverse correlation is often necessary, for example, when an IP host is booted or connected to a network it needs to determine its IP address, unless an address is preconfigured by an administrator. Protocols for such inverse correlations exist in the Internet Protocol Suite. Currently used methods are Dynamic Host Configuration Protocol (DHCP) and, infrequently, inverse ARP.

See also

• Classful network
• Classless Inter-Domain Routing
• Internet Assigned Numbers Authority
• IPv6
• List of assigned /8 IP address blocks
• List of IP protocol numbers
• Regional Internet Registry

External links

• RFC 791 — Internet Protocol
• http://www.iana.org — Internet Assigned Numbers Authority (IANA)
• http://www.networksorcery.com/enp/protocol/ip.htm — IP Header Breakdown, including specific options
• RFC 3344 — IPv4 Mobility
• IPv6 vs. carrier-grade NAT/squeezing more out of IPv4

Address exhaustion:

• RIPE report on address consumption as of October 2003
• Official current state of IPv4 /8 allocations, as maintained by IANA
• Dynamically generated graphs of IPv4 address consumption with predictions of exhaustion dates — Geoff Huston
• IP addressing in China and the myth of address shortage
• Countdown of remaining IPv4 available addresses (estimated)