IPv4 is a data-oriented protocol to be used on a packet switched internetwork (e.g., Ethernet). It is a best effort delivery protocol in that it does not guarantee delivery, nor does it assure proper sequencing, or avoid duplicate delivery. These aspects are addressed by an upper layer protocol (e.g. TCP, and partly by UDP). IPv4 does, however, provide data integrity protection through the use of packet checksums.
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 (~16 million addresses). This reduces the number of addresses that can be allocated as public Internet addresses. As the number of addresses available are consumed, an IPv4 address shortage appears to be inevitable, however Network Address Translation (NAT) has significantly delayed this inevitability.
This limitation has helped stimulate the push towards IPv6, which is currently in the early stages of deployment and is currently the only contender to replace IPv4.
|Notation||Value||Conversion from dot-decimal|
|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|
Most of these formats should work in all browsers. 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.
A final form is not really a notation since it is rarely written in an ASCII string notation. That form is a binary form of the hexadecimal notation in binary. This difference is merely the representational difference between the string "0xCF8E83EB" and the 32-bit integer value 0xCF8E83EB. This form is used for assigning the source and destination fields in a software program.
This created an upper limit of 256 networks. As the networks began to be allocated, this was soon seen to be inadequate.
To overcome this limit, different classes of network were defined, in a system which later became known as classful networking. Five classes were created (A, B, C, D, and E), three of which (A, B, and C) had different lengths for the network field. The rest of an address was used to identify a host within a network, which meant that each network class had a different maximum number of hosts. Thus there were a few networks with each having many host addresses and numerous networks with each only having a few host addresses. Class D was for multicast addresses and Class E was reserved.
Around 1993, these classes were replaced with a Classless Inter-Domain Routing (CIDR) scheme, and the previous scheme was dubbed "classful", by contrast. CIDR's primary advantage is to allow re-division of Class-A, -B and -C networks so that smaller (or larger) blocks of addresses may be allocated to various entities (such as Internet service providers, or their customers) or local area networks.
The actual assignment of an address is not arbitrary. The fundamental principle of routing is that the address of a device encodes information about the device's location within a network. This implies that an address assigned to one part of a network will not function in another part of the network. A hierarchical structure, created by CIDR and overseen by the Internet Assigned Numbers Authority (IANA) and its Regional Internet Registries (RIRs), manages the assignment of Internet addresses worldwide. Each RIR maintains a publicly-searchable WHOIS database that provides information about IP address assignments; information from these databases plays a central role in numerous tools that attempt to locate IP addresses geographically.
|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|
|22.214.171.124/8||Public data networks (per 2008-02-10, available for use)||RFC 1700|
|126.96.36.199/16||Reserved (IANA)||RFC 3330|
|172.16.0.0/12||Private network||RFC 1918|
|188.8.131.52/16||Reserved (IANA)||RFC 3330|
|192.0.0.0/24||Reserved (IANA)||RFC 3330|
|192.0.2.0/24||Documentation and example code||RFC 3330|
|184.108.40.206/24||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|
|220.127.116.11/24||Reserved (IANA)||RFC 3330|
|18.104.22.168/4||Multicasts (former Class D network)||RFC 3171|
|240.0.0.0/4||Reserved (former Class E network)||RFC 1700|
The following are the four ranges reserved for private networks:
|Name||Address range||Number of addresses||Classful description||Largest CIDR block|
|24-bit block||10.0.0.0–10.255.255.255||16,777,216||Single Class A||10.0.0.0/8|
|20-bit block||172.16.0.0–172.31.255.255||1,048,576||16 contiguous Class Bs||172.16.0.0/12|
|16-bit block||169.254.0.0–169.254.255.255||65,536||Single Class B||169.254.0.0/16|
|16-bit block||192.168.0.0–192.168.255.255||65,536||Single Class B||192.168.0.0/16|
The ranges 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 are reserved for private networking by RFC 1918, while the 169.254.0.0/16 range is reserved for Link-Local addressing as defined in RFC 3927.
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.
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.
Today, there are several driving forces for the acceleration of IPv4 address exhaustion:
The accepted and standardized solution is the migration to IPv6. The address size jumps dramatically from 32 bits to 128 bits, providing a vastly increased address space that allows improved route aggregation across the Internet and offers large subnet 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:
As of April 2008, predictions of exhaustion date of the unallocated IANA pool seem to converge to between February 2010 and May 2011
One method to increase both address utilization and security is to use network address translation (NAT). With NAT, assigning one address to a public machine as an internet gateway and using a private network for an organization's computers allows for considerable address savings. This also increases security by making the computers on a private network not directly accessible from the public network.
Since private address ranges are deliberately ignored by all public routers, it is not normally possible to connect two private networks (e.g., two branch offices) via the public Internet. Virtual private networks (VPNs) solve this problem.
VPNs work by inserting an IP packet (encapsulated packet) directly into the data field of another IP packet (encapsulating packet) and using a publicly routable address in the encapsulating packet. Once the VPN packet is routed across the public network and reaches the endpoint, the encapsulated packet is extracted and then transmitted on the private network just as if the two private networks were directly connected.
Optionally, the encapsulated packet can be encrypted to secure the data while it travels over the public network (see VPN article for more details).
RARP is an obsoleted method for translating the hardware address of an interface to its IP address.
| width="4%"||Bits 0–3||4–7||8–15||16–18||19–31|
|0||Version||Header length|| Type of Service|
(now DiffServ and ECN)
|64||Time to Live||Protocol||Header Checksum|
|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.|
Some of the most commonly used protocols are listed below including their value used in the protocol field:
See List of IP protocol numbers for a complete list.
For example, the maximum size of an IP packet is 65,535 bytes while the typical MTU for Ethernet is 1,500 bytes. Since the IP header consumes 20 bytes (without options) of the 1,500 bytes leaving 1,480 bytes of IP data per Ethernet frame (this leads to an MTU for IP of 1,480 bytes). Therefore, a 65,535-byte data payload would require 45 packets (65535/1480 = 44.28).
The reason fragmentation was chosen to occur at the IP layer is that IP is the first layer that connects hosts instead of machines. If fragmentation were performed on higher layers (TCP, UDP, etc.) then this would make fragmentation/reassembly redundantly implemented (once per protocol); if fragmentation were performed on a lower layer (Ethernet, ATM, etc.) then this would require fragmentation/reassembly to be performed on each hop (could be quite costly) and redundantly implemented (once per link layer protocol). Therefore, the IP layer is the most efficient one for fragmentation.
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:
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)|
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)|
Indeed, the amount of data has been preserved — 1480 + 1000 + 1480 + 540 = 4500 — and the last fragment offset 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.
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 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.