IP routing

<h2 id="protocol-classification">Protocol classification</h2>
<a href="/facts/Routing_protocol/LH5U2Qh5">Routing protocols</a> may be broadly distinguished by their realm of operation in terms of network scope. <a href="/facts/Interior_gateway_protocol/RDWHfeLE">Interior gateway protocols</a> are used for routing within <a href="/facts/Autonomous_system_(Internet)/zIVqxPX1">autonomous systems</a>, while <a href="/facts/Exterior_gateway_protocol/p9TqBYxM">exterior gateway protocols</a> route traffic between them. The former group is exemplified by the <a href="/facts/Routing_Information_Protocol/xGeIuGk6">Routing Information Protocol</a> (RIP) and <a href="/facts/Open_Shortest_Path_First/PACpGUHb">Open Shortest Path First</a> (OSPF), while the <a href="/facts/Exterior_Gateway_Protocol/H0HfXRan">Exterior Gateway Protocol</a> (EGP) and the <a href="/facts/Border_Gateway_Protocol/XfYgBzW3">Border Gateway Protocol</a> (BGP) are examples of the exterior type. BGP is the dominant route distribution protocol used on the Internet.

<h2 id="routing-algorithm">Routing algorithm</h2>
The IP forwarding algorithm is a specific implementation of <a href="/facts/Routing/Ml6MVCaC">routing</a> for <a href="/facts/IP_network/KNkxtSpw">IP networks</a>. In order to achieve a successful transfer of data, the algorithm uses a routing table to select a next-hop router as the next destination for a <a href="/facts/Datagram/TMHVctMz">datagram</a>. The <a href="/facts/IP_address/z9V22Sw9">IP address</a> of the selected router is known as the next-hop address.<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a>
The IP forwarding algorithm states:<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a>

Given a destination IP address, D, and network prefix, N:
if ( N matches a directly connected network address )<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a><a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>
Deliver datagram to D over that network link;
else if ( The routing table contains a route for N )
Send datagram to the next-hop address listed in the routing table;
else if ( a default route exists )
Send datagram to the default route;
else
Send a forwarding error message to the originator;
When multiple route table entries match, the entry with the longest <a href="/facts/Subnet_mask/s1rQsNDv">subnet mask</a> is chosen as it is the most specific one.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a> If there are multiple routes with the same subnet mask, the route with the lowest <a href="/facts/Metrics_(networking)/cIlvV1oH">metric</a> is used. If there are multiple default routes, the metric is also used to determine which to use. If there are multiple routes with the same subnet mask and metric, the system may use <a href="/facts/Equal-cost_multi-path_routing/yWnENc41">equal-cost multi-path routing</a> as a forwarding strategy.
When no route is available, an <a href="/facts/Internet_Control_Message_Protocol/VnrV56J0">ICMP</a> error message is sent to the originator of the packet,<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> to inform that host that the packet could not be delivered. To avoid unnecessary <a href="/facts/Retransmission_(data_networks)/Em3pHg0f">retransmission</a> to avoid <a href="/facts/Network_congestion/9ohkRMso">network congestion</a>, the sending host should either stop transmitting or choose another address or route.

<h2 id="routing-table">Routing table</h2>
The following presents a typical <a href="/facts/Routing_table/tufajWvV">routing table</a> in a <a href="/facts/Unix-like_operating_system/YAKrAxzd">Unix-like operating system</a>:

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 71.46.14.1 0.0.0.0 UG 0 0 0 ppp0
10.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 eth0
71.46.14.1 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
172.16.0.0 0.0.0.0 255.240.0.0 U 0 0 0 eth0
192.168.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
192.168.1.0 192.168.96.1 255.255.255.0 UG 0 0 0 eth0
192.168.96.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0

The host has several network interfaces. eth0 is the interface name of the <a href="/facts/Network_interface_card/wyf5FR29">network interface card</a> representing an <a href="/facts/Ethernet/nMfhU8tU">Ethernet</a> port. ppp0 is a <a href="/facts/PPPoE/dSfSRACH">PPPoE</a> interface, which is configured as the <a href="/facts/Default_route/qEc68cgL">default route</a> in this example.
A default route is recognized by the destination 0.0.0.0 and the flag G. A <a href="/facts/Network_router/Nh5cIyVf">network router</a> is identified by the <a href="/facts/Ip_address/z9V22Sw9">network mask</a> 255.255.255.255 and the flag H.

Common routing flags<table><tbody><tr><th>Flag</th><th>Description</th></tr><tr><td>G</td><td>Use Gateway (gateway filled in)</td></tr><tr><td>H</td><td>Target is a Host (<a href="/facts/Bitmask/d9Cu1A1Y">bitmask</a> of 32 bits)</td></tr><tr><td>U</td><td>Route is Up</td></tr></tbody></table>
<h2 id="notes">Notes</h2>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Comer, Douglas E. (2000). Internetworking with TCP/IP (4th ed.). Upper Saddle River, NJ: Prentice Hall. <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Comer, Douglas E. (2000). Internetworking with TCP/IP (4th ed.). Upper Saddle River, NJ: Prentice Hall. <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">"IP Routing and Subnets". Retrieved 2022-08-02. <a href="https://www.eventhelix.com/networking/ip-routing/" target="_blank">https://www.eventhelix.com/networking/ip-routing/</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">"Guide to IP Layer Network Administration with Linux - chapter 4.2. Routing to Locally Connected Networks". Retrieved 2022-08-02. <a href="http://linux-ip.net/html/routing-local.html" target="_blank">http://linux-ip.net/html/routing-local.html</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Many implementations list these directly-connected networks as routing table entries, in which case, this condition is handled by the general route table lookup. <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">"IP Routing and Subnets". Retrieved 2022-08-02. <a href="https://www.eventhelix.com/networking/ip-routing/" target="_blank">https://www.eventhelix.com/networking/ip-routing/</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">"IP Routing and Subnets". Retrieved 2022-08-02. <a href="https://www.eventhelix.com/networking/ip-routing/" target="_blank">https://www.eventhelix.com/networking/ip-routing/</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
</ol>

IP routing open-in-new

IP routing