IP address
From Wikipedia, the free encyclopedia
An
Internet Protocol address (
IP address) is a numerical label assigned to each device (e.g., computer, printer) participating in a
computer network that uses the
Internet Protocol for communication.
[1] An IP address serves two principal functions: host or network interface
identification and location
addressing. Its role has been characterized as follows: "A
name indicates what we seek. An address indicates where it is. A route indicates how to get there."
[2]
The designers of the Internet Protocol defined an IP address as a
32-bit number
[1] and this system, known as
Internet Protocol Version 4 (IPv4), is still in use today. However, because of the growth of the
Internet and the predicted
depletion of available addresses, a new version of IP (
IPv6), using 128 bits for the address, was developed in 1995.
[3] IPv6 was standardized as
RFC 2460 in 1998,
[4] and its
deployment has been ongoing since the mid-2000s.
IP addresses are usually written and displayed in
human-readable notations, such as 172.16.254.1 (IPv4), and 2001:db8:0:1234:0:567:8:1 (IPv6).
The
Internet Assigned Numbers Authority (IANA) manages the IP address space allocations globally and delegates five
regional Internet registries (RIRs) to allocate IP address blocks to
local Internet registries (
Internet service providers) and other entities.
IP versions
Two versions of the Internet Protocol (IP) are in use: IP Version 4
and IP Version 6. Each version defines an IP address differently.
Because of its prevalence, the generic term
IP address typically still refers to the addresses defined by
IPv4. The gap in version sequence between IPv4 and IPv6 resulted from the assignment of number 5 to the experimental
Internet Stream Protocol in 1979, which however was never referred to as IPv5.
IPv4 addresses
In IPv4 an address consists of 32
bits which limits the
address space to
4294967296 (2
32) possible unique addresses. IPv4 reserves some addresses for special purposes such as
private networks (~18 million addresses) or
multicast addresses (~270 million addresses).
IPv4 addresses are canonically represented in
dot-decimal notation,
which consists of four decimal numbers, each ranging from 0 to 255,
separated by dots, e.g., 172.16.254.1. Each part represents a group of 8
bits (
octet) of the address. In some cases of technical writing, IPv4 addresses may be presented in various
hexadecimal,
octal, or
binary representations.
Subnetting
In the early stages of development of the Internet Protocol,
[1]
network administrators interpreted an IP address in two parts: network
number portion and host number portion. The highest order octet (most
significant eight bits) in an address was designated as the
network number and the remaining bits were called the
rest field or
host identifier and were used for host numbering within a network.
This early method soon proved inadequate as additional networks
developed that were independent of the existing networks already
designated by a network number. In 1981, the Internet addressing
specification was revised with the introduction of
classful network architecture.
[2]
Classful network design allowed for a larger number of individual network assignments and fine-grained
subnetwork design. The first three bits of the most significant octet of an IP address were defined as the
class of the address. Three classes (
A,
B, and
C) were defined for universal
unicast
addressing. Depending on the class derived, the network identification
was based on octet boundary segments of the entire address. Each class
used successively additional octets in the network identifier, thus
reducing the possible number of hosts in the higher order classes (
B and
C). The following table gives an overview of this now obsolete system.
Historical classful network architecture
| Class |
Leading
bits |
Size of network
number bit field |
Size of rest
bit field |
Number
of networks |
Addresses
per network |
Start address |
End address |
| A |
0 |
8 |
24 |
128 (27) |
16,777,216 (224) |
0.0.0.0 |
127.255.255.255 |
| B |
10 |
16 |
16 |
16,384 (214) |
65,536 (216) |
128.0.0.0 |
191.255.255.255 |
| C |
110 |
24 |
8 |
2,097,152 (221) |
256 (28) |
192.0.0.0 |
223.255.255.255 |
Classful network design served its purpose in the startup stage of the Internet, but it lacked
scalability in the face of the rapid expansion of the network in the 1990s. The class system of the address space was replaced with
Classless Inter-Domain Routing
(CIDR) in 1993. CIDR is based on variable-length subnet masking (VLSM)
to allow allocation and routing based on arbitrary-length prefixes.
Today, remnants of classful network concepts function only in a
limited scope as the default configuration parameters of some network
software and hardware components (e.g. netmask), and in the technical
jargon used in network administrators' discussions.
Private addresses
Early network design, when global end-to-end connectivity was
envisioned for communications with all Internet hosts, intended that IP
addresses be uniquely assigned to a particular computer or device.
However, it was found that this was not always necessary as
private networks developed and public address space needed to be conserved.
Computers not connected to the Internet, such as factory machines
that communicate only with each other via TCP/IP, need not have globally
unique IP addresses. Three ranges of IPv4 addresses for private
networks were reserved in
RFC 1918. These addresses are not routed on the Internet and thus their use need not be coordinated with an IP address registry.
Today, when needed, such private networks typically connect to the Internet through
network address translation (NAT).
IANA-reserved private IPv4 network ranges
|
Start |
End |
No. of addresses |
| 24-bit block (/8 prefix, 1 × A) |
10.0.0.0 |
10.255.255.255 |
16777216 |
| 20-bit block (/12 prefix, 16 × B) |
172.16.0.0 |
172.31.255.255 |
1048576 |
| 16-bit block (/16 prefix, 256 × C) |
192.168.0.0 |
192.168.255.255 |
65536 |
Any user may use any of the reserved blocks. Typically, a network administrator will divide a block into
subnets; for example, many
home routers automatically use a default address range of 192.168.0.0 through 192.168.0.255 (192.168.0.0/24).
IPv4 address exhaustion
High levels of demand have decreased the supply of unallocated
Internet Protocol Version 4 (IPv4) addresses available for assignment to
Internet service providers and end user organizations since the 1980s. This development is referred to as
IPv4 address exhaustion. IANA's primary
address pool was exhausted on 3 February 2011, when the last five blocks were allocated to the five RIRs.
[5][6] 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, intended to be allocated in a restricted process.
[7]
IPv6 addresses
Main article:
IPv6 address
Decomposition of an IPv6 address from
hexadecimal representation to its binary value.
The rapid exhaustion of IPv4 address space prompted the
Internet Engineering Task Force
(IETF) to explore new technologies to expand the addressing capability
in the Internet. The permanent solution was deemed to be a redesign of
the Internet Protocol itself. This new generation of the Internet
Protocol was eventually named
Internet Protocol Version 6 (IPv6) in 1995.
[3][4] The address size was increased from 32 to 128
bits (16
octets), thus providing up to 2
128 (approximately
3.403×1038) addresses. This is deemed sufficient for the foreseeable future.
The intent of the new design was not to provide just a sufficient
quantity of addresses, but also redesign routing in the Internet by more
efficient aggregation of subnetwork routing prefixes. The resulted in
slower growth of routing tables in routers. The smallest possible
individual allocation is a subnet for 2
64 hosts, which is the
square of the size of the entire IPv4 Internet. At these levels, actual
address utilization rates will be small on any IPv6 network segment.
The new design also provides the opportunity to separate the addressing
infrastructure of a network segment, i.e. the local administration of
the segment's available space, from the addressing prefix used to route
traffic to and from external networks. IPv6 has facilities that
automatically change the routing prefix of entire networks, should the
global connectivity or the routing policy change, without requiring
internal redesign or manual renumbering.
The large number of IPv6 addresses allows large blocks to be assigned
for specific purposes and, where appropriate, to be aggregated for
efficient routing. With a large address space, there is no need to have
complex address conservation methods as used in CIDR.
All modern desktop and enterprise server operating systems include
native support for the IPv6 protocol, but it is not yet widely deployed
in other devices, such as residential networking routers,
voice over IP (VoIP) and multimedia equipment, and network peripherals.
Private addresses
Just as IPv4 reserves addresses for private networks, blocks of
addresses are set aside in IPv6. In IPv6, these are referred to as
unique local addresses (ULA).
RFC 4193
reserves the routing prefix fc00::/7 for this block which is divided
into two /8 blocks with different implied policies. The addresses
include a 40-bit pseudorandom number that minimizes the risk of address
collisions if sites merge or packets are misrouted.
[8]
Early practices used a different block for this purpose (fec0::), dubbed site-local addresses.
[9] However, the definition of what constituted
sites
remained unclear and the poorly defined addressing policy created
ambiguities for routing. This address type was abandoned and must not be
used in new systems.
[10]
Addresses starting with fe80:, called
link-local
addresses, are assigned to interfaces for communication on the attached
link. The addresses are automatically generated by the operating system
for each network interface. This provides instant and automatic
communication between all IPv6 host on a link. This feature is required
in the lower layers of IPv6 network administration, such as for the
Neighbor Discovery Protocol.
Private address prefixes may not be routed on the public Internet.
IP subnetworks
IP networks may be divided into
subnetworks in both
IPv4 and
IPv6. For this purpose, an IP address is logically recognized as consisting of two parts: the
network prefix and the
host identifier, or
interface identifier (IPv6). The
subnet mask or the CIDR prefix determines how the IP address is divided into network and host parts.
The term
subnet mask is only used within IPv4. Both IP
versions however use the CIDR concept and notation. In this, the IP
address is followed by a slash and the number (in decimal) of bits used
for the network part, also called the
routing prefix. For example, an IPv4 address and its subnet mask may be 192.0.2.1 and 255.255.255.0, respectively. The
CIDR notation
for the same IP address and subnet is 192.0.2.1/24, because the first
24 bits of the IP address indicate the network and subnet.
IP address assignment
Internet Protocol addresses are assigned to a host either anew at the
time of booting, or permanently by fixed configuration of its hardware
or software. Persistent configuration is also known as using a
static IP address. In contrast, in situations when the computer's IP address is assigned newly each time, this is known as using a
dynamic IP address.
Methods
Static IP addresses are manually assigned to a computer by an
administrator. The exact procedure varies according to platform. This
contrasts with dynamic IP addresses, which are assigned either by the
computer interface or host software itself, as in
Zeroconf, or assigned by a server using
Dynamic Host Configuration Protocol
(DHCP). Even though IP addresses assigned using DHCP may stay the same
for long periods of time, they can generally change. In some cases, a
network administrator may implement dynamically assigned static IP
addresses. In this case, a DHCP server is used, but it is specifically
configured to always assign the same IP address to a particular
computer. This allows static IP addresses to be configured centrally,
without having to specifically configure each computer on the network in
a manual procedure.
In the absence or failure of static or stateful (DHCP) address
configurations, an operating system may assign an IP address to a
network interface using state-less auto-configuration methods, such as
Zeroconf.
Uses of dynamic address assignment
IP addresses are most frequently assigned dynamically on LANs and broadband networks by the
Dynamic Host Configuration Protocol
(DHCP). They are used because it avoids the administrative burden of
assigning specific static addresses to each device on a network. It also
allows many devices to share limited address space on a network if only
some of them will be online at a particular time. In most current
desktop operating systems, dynamic IP configuration is enabled by
default so that a user does not need to manually enter any settings to
connect to a network with a DHCP server. DHCP is not the only technology
used to assign IP addresses dynamically. Dialup and some broadband
networks use dynamic address features of the
Point-to-Point Protocol.
Sticky dynamic IP address
A
sticky dynamic IP address is an informal term used by cable
and DSL Internet access subscribers to describe a dynamically assigned
IP address which seldom changes. The addresses are usually assigned with
DHCP. Since the modems are usually powered on for extended periods of
time, the address leases are usually set to long periods and simply
renewed. If a modem is turned off and powered up again before the next
expiration of the address lease, it will most likely receive the same IP
address.
Address autoconfiguration
RFC 3330 defines an address block, 169.254.0.0/16, for the special use in
link-local addressing for IPv4 networks. In
IPv6,
every interface, whether using static or dynamic address assignments,
also receives a local-link address automatically in the block fe80::/10.
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.
When the link-local IPv4 address block was reserved, no standards
existed for mechanisms of address autoconfiguration. Filling the void,
Microsoft created an implementation that is called Automatic Private IP Addressing (
APIPA). APIPA has been deployed on millions of machines and has, thus, become a
de facto standard in the industry. In
RFC 3927, the
IETF defined a formal standard for this functionality, entitled
Dynamic Configuration of IPv4 Link-Local Addresses.
Uses of static addressing
Some infrastructure situations have to use static addressing, such as when finding the
Domain Name System (DNS) host that will translate
domain names
to 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
(
RFC 2072).
Conflict
An IP address conflict occurs when two devices on the same local
physical or wireless network claim to have the same IP address - that
is, they conflict with each other. Since only one of the devices is
supposed to be on the network at a time, the second one to arrive will
generally stop the IP functionality of one or both of the devices. In
many cases with modern
Operating Systems,
the Operating System will notify the user of one of the devices that
there is an IP address conflict (displaying the symptom error message
[11][12])
and then either stop functioning on the network or function very badly
on the network, and the user will then be stumped as to how to resolve
the conflict, probably considering the situation an emergency. In some
unfortunate cases, both devices will function very badly on the network.
In severe cases in which one of the devices is the gateway, the network
will be crippled. Since IP addresses are assigned by multiple people
and systems in multiple ways, any of them can be at fault.
[13][14][15][16][17]
Routing
IP addresses are classified into several classes of operational
characteristics: unicast, multicast, anycast and broadcast addressing.
Unicast addressing
The most common concept of an IP address is in
unicast addressing, available in both
IPv4 and
IPv6.
It normally refers to a single sender or a single receiver, and can be
used for both sending and receiving. Usually, a unicast address is
associated with a single device or host, but a device or host may have
more than one unicast address. Some individual PCs have several distinct
unicast addresses, each for its own distinct purpose. Sending the same
data to multiple unicast addresses requires the sender to send all the
data many times over, once for each recipient.
Broadcast addressing
In IPv4 it is possible to send data to all possible destinations
("all-hosts broadcast"), which permits the sender to send the data only
once, and all receivers receive a copy of it. In the IPv4 protocol, the
address 255.255.255.255 is used for local broadcast. In addition, a
directed (limited) broadcast can be made by combining the network prefix
with a host suffix composed entirely of binary 1s. For example, the
destination address used for a directed broadcast to devices on the
192.0.2.0/24 network is 192.0.2.255. IPv6 does not implement broadcast
addressing and replaces it with multicast to the specially-defined
all-nodes multicast address.
Multicast addressing
A
multicast address is associated with a group of interested receivers. In IPv4, addresses 224.0.0.0 through 239.255.255.255 (the former
Class D addresses) are designated as multicast addresses.
[18]
IPv6 uses the address block with the prefix ff00::/8 for multicast
applications. In either case, the sender sends a single datagram from
its unicast address to the multicast group address and the intermediary
routers take care of making copies and sending them to all receivers
that have joined the corresponding multicast group.
Anycast addressing
Like broadcast and multicast,
anycast
is a one-to-many routing topology. However, the data stream is not
transmitted to all receivers, just the one which the router decides is
logically closest in the network. Anycast address is an inherent feature
of only IPv6. In IPv4, anycast addressing implementations typically
operate using the shortest-path metric of
BGP routing
and do not take into account congestion or other attributes of the
path. Anycast methods are useful for global load balancing and are
commonly used in distributed
DNS systems.
Public addresses
A
public IP address, in common parlance, is synonymous with a
globally routable unicast IP address.
[citation needed]
Both IPv4 and IPv6 define address ranges that are reserved for
private networks and
link-local addressing. The term public IP address often used excludes these types of addresses.
Modifications to IP addressing
IP blocking and firewalls
Firewalls perform
Internet Protocol blocking to protect networks from unauthorized access. They are common on today's Internet. They control access to networks based on the IP address of a client computer. Whether using a
blacklist or a
whitelist, the IP address that is blocked is the perceived IP address of the client, meaning that if the client is using a
proxy server or
network address translation, blocking one IP address may block many individual computers.
IP address translation
Multiple client devices can appear to share IP addresses: 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.
[19]
Most commonly, the NAT device maps TCP or UDP port numbers on the
side of the larger, public network to individual private addresses on
the masqueraded network.
In small home networks, NAT functions are usually implemented in a
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 on the Internet.
This type of router allows several computers to share one public IP
address.
Diagnostic tools
Computer operating systems provide various diagnostic tools to examine their network interface and address configuration.
Windows provides the
command-line interface tools
ipconfig and
netsh and users of
Unix-like systems can use
ifconfig,
netstat,
route,
lanstat,
fstat, or
iproute2 utilities to accomplish the task.
See also
References
RFC 760, DOD Standard Internet Protocol (January 1980)
RFC 791, Internet Protocol – DARPA Internet Program Protocol Specification (September 1981)
RFC 1883, Internet Protocol, Version 6 (IPv6) Specification, S. Deering, R. Hinden (December 1995)
RFC 2460, Internet Protocol, Version 6 (IPv6) Specification, S. Deering, R. Hinden, The Internet Society (December 1998)
Smith, Lucie; Lipner, Ian (3 February 2011). "Free Pool of IPv4 Address Space Depleted". Number Resource Organization. Retrieved 3 February 2011.
ICANN,nanog mailing list. "Five /8s allocated to RIRs – no unallocated IPv4 unicast /8s remain".
Asia-Pacific Network Information Centre (15 April 2011). "APNIC IPv4 Address Pool Reaches Final /8". Retrieved 15 April 2011.
RFC 4193 section 3.2.1
RFC 3513
RFC 3879
"Event ID 4198 — TCP/IP Network Interface Configuration". Microsoft. 2009-01-07. Retrieved 2013-06-02. "Updated: January 7, 2009"
"Event ID 4199 — TCP/IP Network Interface Configuration". Microsoft. 2009-01-07. Retrieved 2013-06-02. "Updated: January 7, 2009"
Mitchell, Bradley. "IP Address Conflicts - What Is an IP Address Conflict?". About.com. Retrieved 2013-11-23.
Kishore, Aseem (2009-08-04). "How to Fix an IP Address Conflict". Online Tech Tips Online-tech-tips.com. Retrieved 2013-11-23.
"Get help with "There is an IP address conflict" message". Microsoft. 2013-11-22. Retrieved 2013-11-23.
"Fix duplicate IP address conflicts on a DHCP network". Microsoft. Retrieved 2013-11-23. Article ID: 133490 - Last Review: October 15, 2013 - Revision: 5.0
Moran, Joseph (2010-09-01). "Understanding And Resolving IP Address Conflicts - Webopedia.com". Webopedia.com. Retrieved 2013-11-23.
RFC 5771
