As a Uniform Resource Locator ( URL ) are one type of uniform resource identifiers (URIs). URLs identify and locate a resource by the access method to use (for example, the used network protocol such as HTTP or FTP ) and location (English location ) of the resource in computer networks . In common usage they are also called Internet address designated where it (the colloquial frequent equating of Internet and World Wide Web ^[2] following) usually specially URLs websites are meant.

As the first and most common type of URLs are URIs, the terms are often interchangeably used. The difference is in simple terms is that the URIs for the actual access to resources in the context of the Internet URL syntax thought to the mere naming any resources extend from – so, for example, telephone numbers

scheme

Determines method of the resource should be addressed with any technical. Is generally, but not necessarily identical with the used network protocol over which the resource can be located. Examples include HTTP , HTTPS or FTP , as well mailto (for writing an e-mail) or file (to access local files).

scheme-specific-part

Depending on the scheme are different specific information required, respectively. possible. The following is an example of the HTTP shown for.

user / password

If required, a login of username (username) and password ) are transmitted password (. These are separated by another colon, separating the domain with an at sign ( @ ) prefix.

Although for this example the protocol HTTP was selected, specifying the username and password as part of the URL just not part of the HTTP specification! Current Browser URL syntax accept this though, but ask the user for whether he with the specified data register would really be. The Internet Explorer 6 (Windows XP SP2) is out of line here, by this URL syntax flatly rejected as faulty. With a registry entry you can force him to the same behavior as previous version 5.5 to show his: They take over the login information without permission and pass it directly to the server.

Some other protocols such as FTP , form is an indication of the user data shown in the other hand, entirely correct and that the standards covered by.

host

The host component is in the form of an IPv4 address in decimal notation separated by dots, in the form of an IPv6 address in hexadecimal notation separated by colons and square brackets in or as a FQDN listed.

port

The type of ports allows the control of a TCP / IP ports. If no port is specified, the default port of each protocol used – for example, HTTP 80, HTTPS on 443 and when FTP is 21

url-path

The path describes a particular resource (this may be with the structure of the target system such as cover, or about a file or directory) on the server . The path may be empty. An empty path can be replaced by a slash and is optional for this equivalent.

The interpretation ( file or directory , text file, deliver or script to run ) is left to the server. A typical example of the freedom of interpretation is the behavior at the request of the path / a client: Depending on your server returns the contents of a particularly excellent as the file (like / index.html , / README , / HEADER ), without this the requesting client is apparent. Similarly, the server, however – depending on the protocol – also explicitly associated with this item forward or a directory listing output.

searchPart

In the case of HTTP, after the actual resource indicators – separated by a question mark – a query string to follow. Thus, additional information is transmitted, the server or can be processed on the client side.

fragment

After a double cross , a part of the resource references (typically an anchor in an HTML page, which then automatically heruntergescrollt is: The URL ” # http://example.com/dokument.html absatz3 “would be the third document in the fictitious sales jump.

Examples

• ftp://hans:geheim ftp.example.org @ FTP with user and password
• http://de.wikipedia.org website without a path (called the ” Home “)
• http://de.wikipedia.org/wiki/Uniform_Resource_Locator website with path
• mailto: nobody@example.org for writing an e-mail to the given email address (mail client opens the default with a new, empty mail in which the TO address is already filled)
• news: alt.hypertext display of a Usenet newsgroup (generic, without specifying the network protocol NNTP )
• nntp: alt.hypertext ad to a Usenet newsgroup (with details of the network protocol NNTP)
• telnet: example.org start a Telnet session
• file: / / / C: / temp / test.txt access to a local file

Language

The acronym URL is in the German language usually with a female articles , with a male article uses occasionally. The choice of gender depends on whether it is in line with the German translation of ” the address “) which is feminine (or by the rule that words-or “(here” Locator “) in the German masculine are always” on.

URLs in text

RFC 3986 , Appendix C, recommends URIs (and thus URLs) in texts

• independently on a line
• with double quotes "http://example.com/" or
• with angle brackets <http://example.com/>

against the context and especially against the punctuation of the sentence to distinguish.

History

Name and standardization

In the early days of the WWW (from the end of 1990) was found in the documentation on info.cern.ch first no dedicated name for the addressing of Web pages, the subject was only descriptive than “W3 document address” , “W3 name” , “W3 address “ or “Hyper Text name” . documented at the time specified (and in the first websites used) the form of address equivalent to the but even the later as “URL” standardized form, in the standardization process, although changes were considered because of the now-advanced distribution of the WWW, but again rejected.

In the summer of 1992 tried to Tim Berners-Lee at the IETF meeting in Boston to call into life working group, the documents on the Web should standardize the access. He proposed as names Universal Document Identifier (UDI) prior to, presentation of a general Internet standard should be defined by it under what. But the name was too arrogant “criticism”, mainly on the word “universal” : universal (comprising) was dt. Instead, the group was of the more modest term “uniform” : uniform () dt proposed. Furthermore, was the “Document” by “resource” replaces underscore that the Web should be integrated with other information systems. The URI working group came about after all, and yet another name change for the to be defined standard was adopted: “identifier” was “Locator” replaced emphasize that there are web addresses not permanently registered addresses concerns.

the group was the first – - yet informal manner due to the conflicting standards draft RFC 1630 in June 1994 by Berners-Lee presented. in the first He names names of Berners-Lee the favorite , “Universal Resource Identifiers” in the title and already defined the terms URI, URL and URN. In December 1994, by the group in RFC 1738 standard for “Uniform Resource Locators (URL)” published.

Components

Berners-Lee borrowed some of the individual constituents aware of existing systems to new users as Web addresses directly familiar, respectively. logically appear to be:

• The path part ( http://www.example.com / foo / bar / baz.html ) quotes directly the path syntax in UNIX file systems . ^[20]
• The double-slash notation introduced with the host is from the syntax of the network file system from Apollo Domain / OS , the paths to remote hosts using the model / / example.com / foo / bar / ... have been addressed.
  
• This with a hash of the fragment is marked in the USA standard notation for apartment – and suite numbers in mailing addresses borrowed: “12 Foo Avenue # 34″ stands for “Foo Avenue No. 12, Apartment 34″, and accordingly does foo.html # bar ” part (section, chapter …) bar in the document foo.html . ”

An IP address is an address in computer networks , which – like the Internet – on the Internet protocol IP) based (. You will be assigned devices that are network connected and makes the devices so addressed and so accessible. The IP address can be individual or a group of receivers called a ( multicast , broadcast ). Conversely, a computer multiple IP addresses assigned to it.

The IP address is used, data from the sender to the intended recipient to be able to transport her. Similar to the mailing address on an envelope are data packets provided with an IP address, uniquely identifies the recipient. Because of this address, the “Post”, the router decide the packet should be transported in the direction which. Unlike postal addresses IP addresses are not tied to a specific location.

The best known of today’s common notation IPv4 addresses consist of four numbers, the values ​​of 0-255 and can take from one point to be separated with, for example, 127.0.0.1 . Technically, the address is a 32-digit ( IPv4 ) or 128-digit ( IPv6 ) binary number .

Fundamentals

To communicate between two technical equipment to build one, each of the devices must be able to, the other device to send data. For this data to arrive at the correct remote site, it must be clearly identified (addressed) are. This is done in IP networks with an IP address. Thus, for example, a Web server via a web browser directly addressed by its IP address. The browser asks for this domain name , like “www.example.com”, IP address from any name server , and then speaks directly to the Web server’s IP address “198.51.100.42″ in As.

IP address in IP packets

Each IP packet starts with an information area for the transport through the IP layer , the IP header. This header contains two fields, in which the IP addresses of both the sender and the recipient to be entered before the data packet is sent. The mediation is done on layer 3 in the OSI model , the network layer .

Structure

IPv4

Since the introduction of version 4 of the Internet Protocol used mainly IPv4 addresses consist of 32- bits , ie, 4 octets (bytes) . This 2 are ³² , that is 4,294,967,296 addresses displayed. In dotted decimal notation , the 4 octets than four points separated by decimal integers in the range 0-255 in written,

Example

130.94.122.195 .

IPv6 – new version with a larger address space

Due to the rapidly growing demand for IP addresses is likely that the usable address space of IPv4 will be exhausted sooner or later. Mainly for this reason, IPv6 developed. It uses 128 bits to store addresses so that there are 2 ¹²⁸ = 256 ¹⁶ (= 340,282,366,920,938,463,463,374,607,431,768,211,456 ≈ 3.4 × 10 ³⁸ ) addresses displayed. This number is sufficient for every square millimeter of the surface at least 665.570.793.348.866.944 (= 6.65 × 10 ¹⁷ ) ^[1] provide IP addresses.

Since the decimal ddd.ddd.ddd.ddd.ddd.ddd.ddd.ddd.ddd.ddd.ddd.ddd.ddd.ddd.ddd.ddd and difficult to handle would be confusing, to set IPv6 addresses in hexadecimal dar. To further simplify this presentation, two address octets are combined and the groups represented by separate colon. XXXX: XXXX: XXXX: XXXX: XXXX: XXXX: XXXX: XXXX .

Example

2001:0 db8: 85a3: 08d3: 1319:8 a2e: 0370:7344

Network part and device part

Each IPv4 address is a subnet mask , any IPv6 addresses by specifying the prefix length , in a network and a device component (“host part”) separately. The network mask, ie the prefix, it indicates the address bits are divided on what must be. The network mask of the masked or the prefix length specified bits (network part) in all hosts (computers) of a subnetwork identical. The information whether a device is in the same subnet (ie the same network part of IP address), is a host of needs in order routing to make decisions (see next section).

Example

(Classless) IPv4 Address 130.94.122.195/27

              Decimal Binary calculation 10,000,010 01,011,110 01,111,010 130 094 122 195 IP address 11,000,011           ip-address
network mask 255.255.255.224  11111111 11111111 11111111 111 00 000       AND  netmask
Netzwerkadr. 130 094 122 192  10,000,010 01,011,110 01,111,010 110 00000 = network part

10,000,010 01,011,110 01,111,010 130 094 122 195 IP address 11,000,011           ip-address
network mask 255.255.255.224 11111111 11111111 11111111 111 00 000
                               00000000 00000000 00000000 000 11 111        AND ( NOT  netmask ) Equipment Part 3 00000000 00000000 00000000 000 00 011         = Receptacle

With a netmask of 27 bits set by a network address is 130.94.122.192 . There remain 5 bits and thus 2 ⁵ = 32 addresses for the device component. Of these, nor ever will be an address for the network itself and the broadcast is required, 30 addresses for devices that are available.

Routing

If a device to send a IP packet, the network address parts of the source IP address and destination IP compared. If they match, the destination host is located in the same network and the packet is sent to the recipient directly. In the case of Ethernet networks is the ARP (Address Resolution Protocol) to find the hardware address. The ARP works on the second layer of the OSI model and is establish the connection to the first layer.

If the network part but not identical, so a routing table the IP address of a router (next hop) is searched and sent the packet to this router. This has one or more interfaces in contact with other networks and routes the packet using the same method more – he consulted his part his own routing table and sends the packet to the appropriate next router or to the target. Until the terminal can package many networks and routers go through that. Running through a router is also hop (jump) called the routing method next hop routing .

A router has for each interface its own IP address and subnet mask that belongs to the respective network. Each IP packet is routed separately. The source and destination address in the IP header are set by the station and remain unchanged during the entire journey.

Special IP addresses

Special IPv4 addresses to RFC 3330 :

CIDR address block	Address range	Description	RFC
0.0.0.0 / 8	0.0.0.0 to 0.255.255.255	current network (as a source address is valid)	RFC 3232 (replaced RFC 1700 )
10.0.0.0 / 8	10.0.0.0 to 10.255.255.255	Network for private use	RFC 1918
127.0.0.0 / 8 (1)	127.0.0.0 to 127 255 255 255	Localnet	RFC 3330
169.254.0.0/16	169.254.0.0 to 169 254 255 255	Zeroconf	RFC 3927
172.16.0.0/12	172.16.0.0 to 172.31.255.255	Network for private use	RFC 1918
192.0.0.0/24	192.0.0.0 to 192.0.0.255	reserved, but provided for the award
192.0.2.0/24	192.0.2.0 to 192.0.2.255	Documentation and sample code ( TEST-NET-1 )	RFC 5737 (replaced RFC 3330 )
192.88.99.0/24	192.88.99.0 to 192.88.99.255	6to4 – Anycast -Weiterleitungspräfix	RFC 3068
192.168.0.0/16	192.168.0.0 to 192.168.255.255	Network for private use	RFC 1918
198.18.0.0/15	198.18.0.0 to 198.19.255.255	Network benchmark tests	RFC 2544
198.51.100.0/24	198.51.100.0 to 198.51.100.255	Documentation and sample code ( TEST-NET-2 )	RFC 5737
203.0.113.0/24	203.0.113.0 to 203.0.113.255	Documentation and sample code ( TEST-NET-3 )	RFC 5737
224.0.0.0 / 4	224.0.0.0 to 239.255.255.255	Multicasts (former Class D network )	RFC 3171
240.0.0.0 / 4	240.0.0.0 to 255.255.255.255	Reserved (former Class E network)	RFC 3232 (replaced RFC 1700 )
255 255 255 255 2)	255.255.255.255	Broadcast

After fulfilling this list 622 199 809 of around 4.3 billion IPv4 addresses, or 14.5% of all possible IPv4 addresses a particular purpose.

1. The network 127.0.0.0 / 8 refers to the local machine ( loopback address ). That power is often the area address 127.0.0.1 with the hostname localhost responsive. Addresses from this range are used to communicate a client with a server process on the same computer. With command line commands such as ssh localhost or ftp 127.0.0.1 , the local server computer to be addressed one on their functioning in order to test such.
2. The special address 255.255.255.255, the highest device address in the network also be used as a broadcast address next. This network parameters is sending broadcasts without knowledge of other possible. This is for protocols such as BOOTP and DHCP important.

Thus there are three IP address types:

• Unicast : Send to an individual on the Internet (normal addressing).
• Broadcast : send to all devices on the same network ( subnet ). This is in IPv6 Multicast replaced.
• Multicast : Send some products on the same network (or devices in the multicast backbone network ).

No longer reserved IP addresses

With the RFC 5735 addresses were about 50 million IP-enabled. The reservation of the following address ranges has been lifted and released for distribution.

CIDR address block	Address range	Number	Description	RFC
14.0.0.0 / 8	14.0.0.0 to 14,255,255,255	16.777.216	Public data network	RFC 3232 (replaced RFC 1700 )
24.0.0.0 / 8	24.0.0.0 to 24,255,255,255	16.777.216	Cable Television Networks
39.0.0.0 / 8	39.0.0.0 to 39,255,255,255	16.777.216	In January 2011, the APNIC allocated	RFC 1797
128.0.0.0/16	128.0.0.0 to 128.0.255.255	65.536	reserved, but provided for the award
191.255.0.0/16	191.255.0.0 to 191.255.255.255	65.536	reserved, but provided for the award	RFC 1918
223.255.255.0/24	223.255.255.0 to 223 255 255 255	256	reserved, but provided for the award	RFC 3330

DNA – translation of computer names to IP addresses

On the globally available domain name system (DNS) names to IP addresses can (and vice versa) are resolved. The name www.example.com is for example the IPv4 address 208.77.188.166 , the name www.ipv6.uni-muenster.de the IPv6 address 2001:638:500:101:2 e0: 81ff: ​​fe24: 37c6 .

Assignment of IP addresses and network areas

IANA – Internet Assigned Numbers Authority

The assignment of IP networks in the Internet is used by the IANA regulated. In the early years of the Internet networks were IPv4 addresses, or in large blocks from the IANA to organizations, companies or universities awarded directly. For example, the area was the 13.0.0.0 / 8 and therefore 16,777,216 addresses of Xerox Corporation allocated. Merck & Co. received from the IANA also an area of 16,777,216 addresses ( 54.0.0.0 / 8 ), as well as IBM ( 9.0. 0.0 / 8 ). The only German company which has a / 8 range has been allocated, the debis AG ( 53.0.0.0 / 8 ). Today, the IANA allocates blocks to regional registries.

RIR – Regional Internet Registry

Since February 2005 there are five regional registries, the Regional Internet Registries ) are called RIR (:

• AfriNIC (African Network Information Centre) – responsible for Africa
• APNIC (Asia Pacific Network Information Centre) – responsible for the region Asia-Pacific
• ARIN (American Registry for Internet Numbers) – North America
• LACNIC (Regional Latin American and Caribbean IP Address Registry) – Latin America and Caribbean
• RIPE NCC (Réseaux IP Européens Network Coordination Centre) – Europe , Middle East , Central Asia .

LIR – Local Internet Registry

The Local Internet Registries (LIR) said local authorities give the award to them from the RIRs allocated addresses to their customers. The object of the LIR usually meet Internet Service Provider . LIR customers can either retail or other (sub) providers are.

The addresses of customers can be either permanently assigned to the ( static IP address, fixed IP address ), and Internet connection are dynamically allocated structure for the ( dynamic IP address, dynamic IP address ). Permanently assigned addresses are used primarily with dedicated lines used, or if server address should be operated on the IP.

Which customers or any Local Internet Registry IP address or a network has been assigned, can be on the Whois databases of the RIRs to determine.

Private networks

In private, local networks ( LAN ) allows one IP addresses are assigned. But should the IPv4 addresses from the for RFC 1918 called private networks are used (for example, 192.168.1.1, 192.168.1.2, ... ). These addresses are not assigned by the IANA and the Internet continue to not routed. In order to still allow an internet connection to be in a router using Network Address Translation that LAN internal addresses public, valid in the Internet IPv4 addresses translated to. For packages to arrive addressed to the public address, public address is in turn translated back into private address.

Net classes

Originally, IPv4 addresses have been in network classes A to C with different network masks divided by. Classes D and E were designed for specific tasks. Due to recent increases in routing tables, routing was 1993, the classless CIDR ( Classless Inter-Domain Routing ) is introduced. So that it does not matter which network an IPv4 address class belongs.

Device configuration

Manual configuration

For administrators there are programs to display the IP address and configure. unixoid operating systems use this command, the ifconfig for Linux, ip available, DOS or Windows use, depending on the version, type ipconfig or winipcfg .

Examples

The network interface eth0 is assigned the IPv4 address 192.168.0.254 in a / 27 subnet.

• Unix (FreeBSD, Mac OS X): ifconfig eth0 192.168.0.254/27
• Linux : ip addr add 192.168.0.254/27 brd + dev eth0
• Linux (old): ifconfig eth0 192.168.0.254 broadcast 192.168.0.255 netmask 255 255 255 224

The part “broadcast 192.168.0.255″ or “brd +” are optional. (“Brd +” stands for the automatic calculation of the broadcast address, it can also be given a specific address. Ifconfig calculates the broadcast address in newer versions automatically, eg from FreeBSD 4.x).

Automatic configuration

Through protocols such as BOOTP or DHCP IP addresses when booting the computer by a corresponding server assigned. On the server administrator to a range of IP addresses defined, from which other computers can see an address at startup. This address is to the computer lease . Computers that may need fixed addresses, the Ethernet network via its MAC address to identify and obtain a permanent address.

The advantage here is the central management of the addresses. If after installing the operating system provides for the automatic configuration, no further settings for network access will be made ​​longer. Mobile devices such as laptops can share addresses, if not all devices are connected to the network simultaneously. In addition, they can not change the configuration as needed in different networks sample company, customer networks, home networks) are integrated to the (.

For IPv6, there is also the possibility of auto configuration , the server does not need.

Dynamic addressing

If a host connection with a new network a new IP address is assigned to each, it is called dynamic addressing . In the LAN area, the dynamic addressing via DHCP widespread. In the Internet sector is dynamic addressing mainly by Internet Service Providers (ISP) used the Internet access via dial-up lines to offer. They use dynamic addressing via PPP or PPPoE .

Advantage of dynamic addressing, is that far fewer than one IP address per customer needs is the average, there are never any customers online simultaneously. A ratio of 1:10 to 1:20 is common. The RIPE NCC requires its LIRs evidence of the use of their assigned IP addresses. A fixed allocation of addresses is accepted only in justified cases, for example for the operation of servers, or for accounting purposes.

With DSL connection of customer to use the official provider usually also dynamically assigned IPs. Due to the common practice that the DSL router provider associated with the remains always (24 hours a day, possibly with forced separation and automatic reconnection), usually is the corresponding router always assign the same IP (see DynDNS ).

Static addressing

Static addressing is basically used in applications where dynamic addressing is not technically possible or not meaningful. Thus obtained in LANs, for example, gateways , servers and network printers usually fixed IP addresses. In the Internet access area is for static addressing in particular router on dedicated lines used. Also for Machine-to-machine communications, especially in mobile communications ( GPRS ) are increasingly static addressing. Static addresses are usually configured manually, but also has automatic addressing (see above) are assigned.

Enter multiple addresses on a network card

Usually, each network interface (eg network card) of a host to an IPv4 address assigned exactly. In some cases (see below) however, it is necessary to interface more IPv4 addresses to assign one. This is known as IP aliasing means. A number of IPv4 addresses on one NIC others used in to several of the same services it to operate in parallel in order from different subnets reachable to make host one or a service logically be separated from the host so that he – with its IPv4 address and transparent Hardware can be moved – to another client for the.

Example (FreeBSD)

The network interface fxp0 get the IPv4 address of 192.168.2.254 with a / 26 subnet as an alias

ifconfig fxp0 alias 192.168.2.254 netmask 255 255 255 192

On Linux, just the same command as above

ip addr add 192.168.2.254/24 dev eth0

used to add more addresses.

For IPv6, the binding of multiple addresses to a network interface the rule, for example, a link-local operators to find a global address and dynamically assigned prefixes in addition to fixed or IPv6 addresses on the same host multiple Internet providers to have available. Furthermore, the above reasons apply as for IPv4.

Different networks on one physical network

On a physical network (eg Ethernet), different networks (with different network address part) is placed and used simultaneously. This will be used, inter alia, if later, the network should be split up or if previously separate networks are combined.

A tree is in the graph theory , a special graph , with a single hierarchy can be modeled. Depending on whether the edges of the tree significant direction have one, can be graph-theoretical trees divide in undirected trees and rooted trees , and rooted trees in Out-Trees , where the edges from the root go in, and In-Trees , where edges in the direction of root showing.

In the computer science trees are often used as data structure used. In this case, but they are otherwise represented as a general graph. By removing an edge tree falls into two sub-trees and thus forms a so-called forest .

Special trees

There are a variety of terms to specify the trees closer. There are, for example

• empty trees (they do not contain nodes and edges),
• Binary trees ,
• Binomial trees ,
• balanced trees and
• complete binary trees .

Equivalent characterizations of trees

A graph G = (V, E) with | V | = n nodes and | E | = m edges can be defined as a tree equivalent to statements by the following:

• Between two nodes of G there is exactly one path.
• G is connected and it is m = n-1 . (There is always an edge is less than node.)
• G contains no cycle and we have m = n-1 .
• G is (minimally) coherent and (maximum) acyclic.

Trees as a data structure

Rooted trees, especially out-Trees are often used as a data structure. In case of restricted procedure can be implemented so that each node has a fixed set of variables or an array of references to his children contains. Often have the node and a reference to its parent node ( back pointer ). A tree of unlimited order can be implemented by dynamic arrays instead of lists used. In programming languages ​​without dynamic lists process has also proven, in which a general tree by a binary tree is implemented:

The red line marks the realized common tree while the arrows represent the actual data structures. The basic principle is that each is a pointer to the leftmost son shows, the other to the right brother refers. Here, nodes may be a direct access to individual-specific son no longer possible, since one of siblings who have to work ahead. But this implementation is very memory efficient.

Drawing trees

The graphical output of a tree is a nontrivial problem. The general rule is that every planar tree, ie, drawn without overlapping the edges. Depending on the application, there are more requests to the kind of output:

• all edges are straight lines
• all nodes have integer coordinates
• the smallest possible footprint with possible aesthetic result
• all edges from father to child is strictly monotonic decreasing

There are several algorithms whose results look quite different. They usually only solve some but not all wishes to the output. Known algorithms are the HV-trees (horizontal-vertical) and the algorithm of Walker .

Number of trees

There are n ^{n – 2} different labeled trees with n nodes. This statement is a set of Cayley known.