IP address

 

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An IP address (Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. Any participating network device—including routers, switches, computers, time-servers, printers, Internet fax machines, and some telephones—can have their own unique address.

In other words, the IP address acts as a locator for one IP device to find another and interact with it. It is not intended, however, to act as an identifier that always uniquely identifies a particular device.

An IP address can also be thought of as the equivalent of a street address or a phone number (compare: VoIP (voice over (the) internet protocol)) for a computer or other network device on the Internet. Just as each street address and phone number uniquely identifies a building or telephone, an IP address can uniquely identify a specific computer or other network device on a network. An IP address differs from other contact information, however, because the linkage of a user's IP address to his/her name is not publicly available information.

Further, an IP address is not necessarily linked, in a persistent way, to a physical location or even data link layer address.

In the past, an IP address could be considered a unique identifier of a particular IP host, in addition to being a locator. When it was usable as an identifier, it was static, and it was assumed to be globally unique from end to end of the Internet.

In current practice, an IP address is less likely to be an identifier, due to technologies such as:

• Dynamic assignment, as with an address that is assigned by the access device by which the user's host connects over a dialup telephone line or by a set-top box for an IP over cable network. However the network provider maintains a database of which IP address was assigned to which access port on dialup, or MAC address on LANs or broadband networks. This information, assuming it is available to the investigator, may help to identify the computer, although that is unlikely if it was a dialup connection where the identifier is of the dial-in port, not the computer itself. More extensive forensic work, with access to telephone records, may identify the calling telephone, although that may itself be a "cutout" on the way to the real telephone.
• Network address translation (or NAT), a feature common on gateway routers in corporate networks or home LANs, where the address visible to the Internet is the "outside" of a device that maps it to a completely different and hidden address on the "inside". See IP Address Translation, below.

IP Addressing

A machine may be connected by millions of other machines across the internet, and there is no single cable connecting all those computers ; but there is a logical connection in the sense that you can use the telnet program from a machine at California and connect by a machine at Germany : but how do the packets get from one place by another? How is the local network at California prevented being overloaded of packets which are being sent from many machines at Germany, yet also ensuring that these telnet packets do get through the local network at California? The answer is provided by the Internet Protocol (IP).

A street address is not always sufficient to ensure delivery by a letter, and likewise the IP is not always sufficient to ensure packet delivery. If I send you a letter, I might not do so via a single, omnipotent central post office, whose job it be to distribute mail throughout the entire country. Because of the incredibly large numbers of items, this would be impractical. Instead, there are thousands of offices, all over the country, whose job it is to route the mail for us.

If we live within a small town, the local post office will receive and route a letter which is destined by a local address before it goes further ; and if we do not all live within the same town so mail which is addressed by outside of the town will be sent by another post office by processing.

A similar situation applies by IP addresses. At local self-contained networks, the IP address alone is sufficient so to specify a destination ; but where many networks are combined, a machine spends more time trying to deduce whether a packet belongs by itself rather than actually processing the information contained within the packet. So a network mask is used. Similarly than a zip code instructs a postal worker how to process an item of mail, whether locally or otherwise, the network mask (or netmask) tells a machine whether it can ignore a packet or whether it needs to process the packet further.

Each machine on the network must have its own, unique IP address. Similarly than each house has a unique mail address, if a network is connected by the rest of the world, that address must not only be unique within the local network, but unique within the rest of the world, also. By the most common IP version (IPv4), IP addresses are 32-bit values, which are represented usually by four sets of numbers, ranging from 0-255 separated by dots (.). This is referred by as dotted-decimal notation. By dotted-decimal notation, an address might look like this:

147.132.42.18

Because all these numbers range between 0-255, those can be represented by eight bits and are therefore referred by as an octet. This IP address is often thought being composed as a network portion (at the beginning) and a node (or machine) portion at the end. This would be comparable by writing a street address as:

95061.Main_Street.42

where 95061 is the zip code and Main Street is the street and 42 is the house number at that street. The reason why we write the IP address by this fashion is intuitive and obvious.

Currently there are three classes of networks used commonly, and these three may be segregated by both the number range used by the first octet, and also by the number of octets which are used so to identify a network. Class A networks are most common and implement the first octet of the network address. Networks of class A will have the first octet within the range of 1-126. Class B networks use the first two octets, the first being in the range of 128-192. The smallest networks are class C networks, and use the first three octets of the network address and implement the first within the range of 192-223. How IP addresses are segregated by different network classes is shown in this table.

Table - IP Addresses : Hosts by Network

Class	Range within 1st octet	Network ID	Host ID	Possible number of networks	Possible number of hosts
A	1-126	a	b.c.d.	126	16,777,214
B	128-191	a.b	c.d	16,384	65,534
C	192-223	a.b.c	d	2,097,151	254

There are a couple of things to illustrate about this table. Firstly, the network address as 127 represents the local computer, regardless than what network it is really at. This is helpful by testing as well than many internal operations. The network addresses as 224 and above are reserved by special purposes : such as multicast addressing. The terms "possible networks" and "possible hosts per network" are calculated mathematically. In some cases, 0 and 255 are not acceptable values by neither the network addresses nor the host addresses. However, 0 can be used as a number value inside of an IP address by either the second or third octet (by example, 10.2.0).

A Class A address does not necessarily imply that there are 16 million hosts on a single network. This would be impossible to administrate, and would excessively burden most network technologies. What normally happens is that a single company, such as Hewlett-Packard is assigned a Class A address, and Hewlett-Packard subsequently segregates this further into smaller sub-nets.

A network host uses the network ID and host ID so to determine which packets it should either receive or ignore, and a network host also uses the network ID and host ID so to determine that only nodes which have the same network ID will accept each other's IP-level broadcasts. Because the IP address of the sender is included onto all outgoing IP packets, it should be useful that the receiving computer must derive from where it came by scrutinizing the network ID and host ID inside of the IP address field : this is done by using subnet masks.

In some cases, it is not necessary that there be an unique IP address within the world, because the network shall be never connected by the rest of the outside world. By example, at a factory where the machines communicate with each other via TCP/IP, there would not be reason that these machines be accessible by and from the Internet. Therefore, there not be any need by them each to have a unique IP address officially.

You could assign randomly IP addresses by these machines and merely hope that your router is configured not to route the packets from these machines correctly. However, one mistake and there is the potential to render inoperable both your own network, and someone else's also.

The solution was provided by RFC-1918. Here, three sets of IP address were defined by usage within "private" networks, and these three sets of IP addresses won't be routed, and therefore there is no need to co-ordinate the operation to those by any of the registrative agencies. The IP addresses are:

10.0.0.0 - 10.255.255.255 172.16.0.0 - 172.31.255.255 192.168.0.0 - 192.168.255.255

Discernibly there is a single class A network address with many hosts, and also 16 class B network addresses, and 255 class C networks also. Therefore, no matter what size your network is, you can find a private network to suit your needs.

IP versions

The Internet Protocol has two versions currently in use (see IP version history for details). Each version has its own definition of an IP address. Because of its prevalence, "IP address" typically refers to those defined by IPv4.

IP version 4

Main article: IPv4#Addressing

IPv4 only uses 32-bit (4 bytes) addresses, which limits the address space to 4,294,967,296 (2³²) possible unique addresses. However, many 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 be allocated as public Internet addresses, and as the number of addresses available is consumed, an IPv4 address shortage appears to be inevitable in the long run. 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.

Example: 127.0.0.1 (Loopback)

IP version 6

Main article: IPv6#Addressing

IPv6 is the new standard protocol for the Internet. Windows Vista, Apple Computer's Mac OS X, and an increasing range of Linux distributions include native support for the protocol, but it is not yet widely deployed elsewhere.

Addresses are 128 bits (16 bytes) wide, which, even with a generous assignment of netblocks, will more than suffice for the foreseeable future. In theory, there would be exactly 2¹²⁸, or about 3.403 × 10³⁸ unique host interface addresses. Further, this large address space will be sparsely populated, which makes it possible to again encode more routing information into the addresses themselves.

Example: 2001:0db8:85a3:08d3:1319:8a2e:0370:7334

One source^[1] notes that there will exist "roughly 5,000 addresses for every square micrometer of the Earth's surface". This enormous magnitude of available IP addresses will be sufficiently large for the indefinite future, even though mobile phones, cars and all types of personal devices are coming to rely on the Internet for everyday purposes.

The above source, however, involves a common misperception about the IPv6 architecture. Its large address space is not intended to provide unique addresses for every possible point. Rather, the addressing architecture is such that it allows large blocks to be assigned for specific purposes and, where appropriate, aggregated for providing routing. With a large address space, there is not the need to have complex address conservation methods as used in classless inter-domain routing (CIDR).

IP version 6 private addresses

Just as there are addresses for private, or internal networks in IPv4 (one example being the 192.168.0.0 - 192.168.255.255 range), there are blocks of addresses set aside in IPv6 for private addresses. Addresses starting with FE80: are called link-local addresses and are routable only on your local link area. This means that if several hosts connect to each other through a hub or switch then they would communicate through their link-local IPv6 address.

Early designs specified an address range used for "private" addressing, with prefix FEC0. These are called site-local addresses (SLA) and are routable within a particular site, analogously to IPv4 private addresses. Site-local addresses, however, have been deprecated by the IETF, since they create the same problem that does the existing IPv4 private address space (RFC 1918). With that private address space, when two sites need to communicate, they may have duplicate addresses that "combine". In the IPv6 architecture, the preferred method is to have unique addresses, in a range not routable on the Internet, issued to organizations (e.g., enterprises).

The preferred alternative to site-local addresses are centrally assigned unique local unicast addresses (ULA). In current proposals, they will start with the prefix FC00.

Neither ULA nor SLA nor link-local address ranges are routable over the internet.

IP address subnetting

Main article: Subnetwork

Both IPv4 and IPv6 addresses utilize subnetting, or dividing the IP address into two parts: the network address and the host address. By using a subnet mask, the computer can determine where to split the IP address.

Static and dynamic IP addresses

When a computer uses the same IP address every time it connects to the network, it is known as a Static IP address. In contrast, in situations when the computer's IP address changes frequently (such as when a user logs on to a network through dialup or through shared residential cable) it is called a Dynamic IP address.

Method of assigning

Static IP addresses are manually assigned to a computer by an administrator, either through the operating system configuration or through a command (e.g. ipconfig or ifconfig). This contrasts with dynamic IP addresses, where an IP address is automatically assigned to a computer by a remote server which is acting as a Dynamic Host Configuration Protocol server. Even through IP addresses assigned using DHCP may stay the same for long periods of time, they are liable to change depending on the addresses available in the set scope.

Uses of dynamic addressing

Dynamic IP Addresses assigned, on LANs or most broadband networks, by Dynamic Host Configuration Protocol (DHCP) servers are used because it reduces the administrative burden of assigning static addresses within a network. In most desktop operating systems, dynamic IP configuration is enabled by default. Dialup and some broadband networks do not use DHCP, but the dynamic IP addressing capability of the Point-to-Point Protocol.

Uses of static addressing

Static addressing is essential in some infrastructure situations, such as finding the Domain Name Service directory host that will translate domain names to numbers (IP addresses). Static addresses are also convenient, but not absolutely necessary, to locate servers inside an enterprise. An address obtained from a DNS server comes with a time to live, or caching time, after which it should be looked up to confirm that it has not changed. Even static IP addresses do change as a result of network administration, however (RFC 2072).

Modifications to IP addressing

IP blocking and firewalls

Main articles: IP blocking and Firewall

Firewalls are common on today's Internet. For increased network security, they allow or deny access to their private network based on the public IP of the client. Whether using a blacklist or a whitelist, the IP address that is blocked is the perceived public IP address of the client, meaning that if the client is using a proxy server or NAT, blocking one IP address might block many individual people.

IP address translation

Main article: network address translation

IP addresses can appear to be shared by multiple client devices either because they are part of a shared hosting web server environment or because an IPv4 network address translator (NAT) or proxy server acts as an intermediary agent on behalf of its customers, in which case the real originating IP addresses might be hidden from the server receiving a request. A common practice is to have a NAT hide a large number of IP addresses in a private network. Only the "outside" interface(s) of the NAT need to have Internet-routable addresses.

Most commonly, the NAT device maps TCP or UDP port numbers on the outside to individual private addresses on the inside. Just as there may be site-specific extensions on a telephone number, the port numbers are site-specific extensions to an IP address.

In small home networks, NAT functions are usually performed by an residential gateway device, typically one marketed as a "router". In this scenario, the computers connected to the router would have 'private' IP addresses and the router would have a 'public' address to communicate with the Internet. This type of router allows several computers to share one public IP address.

IP addresses are managed and created by the Internet Assigned Numbers Authority (IANA). The IANA generally allocates super-blocks to Regional Internet Registries, who in turn allocate smaller blocks to Internet service providers and enterprises.

See also

External links

• IP at the Open Directory Project
• Articles on CircleID about IP addressing
• IP-Address Management on LANs — article in Byte magazine
• Understanding IP Addressing: Everything You Ever Wanted To Know
• How to get a static IP address - clear instructions for all the major platforms
• An exception, how to put same IP address on two computers
• How to decipher octal (leading zero) IP addresses (scroll down to the "Another URI that has been used ---" paragraph)
• Get full info about own or given IP address

RFCs

• IPv4 addresses: RFC 791, RFC 1519, RFC 1918, RFC 2071, RFC 2072
• IPv6 addresses: RFC 4291, RFC 4192

References

1. ^ Article in MIT magazine Technology Review introducing IPv6

ksh:IP-Nummerkab:Tansa IPbat-smg:IP adrėsas

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