In computer networking, a routing table is an electronic table or database type object that is stored in a router or a networked computer. The routing table stores the routes (and details related with those routes) and network destinations. The routing table contains the topology of the immediate network. The primary goal of routing protocols and static routes is to construct the routing tables.
Using a routing table can be seen as using a map in package delivery. Whenever one networked node needs to send data to another networked node, it needs to know where to send it. Whenever a node cannot connect directly to its destination node, it needs to find another way to send the data packets. Whenever a node does not know where to send the data packets, it sends an IP packet to a gateway in the Local network. Since it is a complex task to route the data packets to the correct destination, a network gateway needs to keep track of the destination. A Routing Table is the method in which this data is stored, similar to a map. It is a database which keeps track of the network paths and provides this information to the node requesting the data.
In hop-by-hop routing, each routing table lists the addresses reachable destinations and the address of the next device along the path to that destination (the next hop). Assuming that the routing tables are consistent, the simple algorithm of relaying packets to their destination's next hop is adequate to transmit the data to any node in the network. Hop-by-hop is the elementary characteristic of the IP Layer.
The three types of entries in a routing table:
• Network Route: A route (or a path) to a specific node on the local network or the Internet.
• Host Route: A route to a specific network destination (Network ID and Host ID). Host routes enable intelligent routing assessments to be made for sending the network data packets. Host routes are also used to create custom routes to control and optimize specific types of network traffic.
• Default Route: A route which is used when no other appropriate routes for the destination are found in the routing table. If a router or computer system, cannot find an appropriate route for the destination, the default route is used.
Displaying the Routing Table
Under both Microsoft Windows and Unix, the `netstat -r` command will display the routing table. Almost always, the syntax `netstat -nr` is used. The 'n' tells `netstat` do show IP addresses instead of doing name resolution and showing host names.
Sample Microsoft Windows Routing Table
C:>netstat -nr

Route Table
===================================================
Interface List
0x1 ........................... MS TCP Loopback interface
0x2 ...00 11 43 64 59 3e ...... Broadcom 570x Gigabit Integrated Controller - Packet Scheduler Miniport
0x10004 ...00 0c f1 56 98 ad ...... Intel(R) PRO/Wireless LAN 2100 3A Mini PCI Adapter - Packet Scheduler Miniport
===================================================
===================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 192.168.1.1 192.168.1.100 30
127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
192.168.1.0 255.255.255.0 192.168.1.100 192.168.1.100 30
192.168.1.100 255.255.255.255 127.0.0.1 127.0.0.1 30
192.168.1.255 255.255.255.255 192.168.1.100 192.168.1.100 30
224.0.0.0 240.0.0.0 192.168.1.100 192.168.1.100 30
255.255.255.255 255.255.255.255 192.168.1.100 2 1
255.255.255.255 255.255.255.255 192.168.1.100 192.168.1.100 1
Default Gateway: 192.168.1.1
===================================================
Persistent Routes:
None
Sample Unix Routing Table
$ netstat -nr
Routing tables

Internet:
Destination Gateway Flags Refs Use Netif Expire
default 10.0.0.1 UGS 2 146537336 fxp0
127.0.0.1 127.0.0.1 UH 0 3414961 lo0
10.0.0.1/26 link#1 UC 0 0 fxp0
10.0.0.2 00:02:b3:4c:65:27 UHLW 0 255732 lo0
10.0.0.1 00:02:7d:cc:3d:00 UHLW 1 0 fxp0 1087
Altering the Routing Table
A dynamic routing protocol such as BGP or OSPF will make automated changes to the system's routing table.
Manual changes to a routing table can be made by adding a static route.
A static route is a manually defined path for packets to travel to a destination across the internetwork.
Under both Microsoft Windows and Unix, static routes are created using the `route` command. The syntax of the `route` commands differs slightly between the two Operating Systems.
The example shows the creation of a static route for a single host:
# route add 192.168.1.100 10.0.0.1
add host 192.168.1.100: gateway 10.0.0.1
We can display the route which this command created using the `netstat -nr command:
# netstat -nr
Routing tables

Internet:
Destination Gateway Flags Refs Use Netif Expire
..
192.168.1.100 10.0.0.1 UGHS 0 0 fxp0
..
Static routes are often used to create null routes.
Books on Routing

Routing TCP/IP Volume I
This all-encompassing TCP/IP text defines completeness with its one-thousand-plus pages and hundreds of configuration screens, exercises, charts, tables, and references. Though initially overwhelming, such thoroughness is what's demanded of those who ultimately pass the Cisco Certified Internetwork Expert (CCIE) exam, the bruising test for which Routing TCP/IP, Volume 1 serves as preparatory material.
Such a goal is evidenced in much of the three-part book's design. For example, nearly every chapter includes some combination of recommended readings, review questions, configuration and troubleshooting exercises, case studies, and summaries. In addition, there are numerous appendices devoted to answers to questions and exercise solutions as well as one that outlines exam preparation tips.
Doyle begins this volume with a review of several basic topics, including a chapter that outlines the parameters of TCP/IP itself. Though some may already be familiar with that material, the sections on dynamic and static routing serve as valuable review material for readers of all levels. The second part deals with TCP/IP interior routing protocols, such as Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), and Open Shortest Path First (OSPF). The book ends on a challenging note with explanations of route redistribution, route filtering, and the like. Routing TCP/IP is required reading for those readers with the ambitious goal to both understand the protocol and excel on the CCIE exam.

IP Routing Fundamentals
A comprehensive ntroduction to routing concepts and protocols in IP networks.Comprehensive review of the operational mechanics of today's leading routing protocols, including IGRP, EIGRP, OSPF, RIP, and RIP-2. Detailed explanation of IP addressing, including classful and classless addresses, subnetting, supernetting, Classless Interdomain Routing (CIDR), and Variable Length Subnet Masks (VLSM). Side-by-side comparisons of various LAN segmentation technologies, including bridges, switches, and routersExploration of how routers are used to build wide area networksExamination of the future of routing, including IPv6, next generation routing protocols, host-based routing, and IP SwitchingIP Routing Fundamentals is the definitive introduction to routing in IP networks. This comprehensive tome explores the mechanics of routers, routing protocols, network interfaces, and operating systems. This reference provides essential background information for network professionals who are deploying and maintaining LANs and WANs, as well as IT managers who are seeking information on how evolving internetworking technology will affect future networks. Part I discusses the many roles routers play in networks, Part II talks about the inner working of routers, Part III works with the operational issues of routing protocols, and Part IV addresses implementation issues that provide practical insight, in addition to a discussion of the future of routing.

Teach Yourself Routing in 24 Hours
Learn routing in the most efficient, enjoyable way possible with Sams Teach Yourself Routing in 24 Hours. Book presents routing concepts in a vendor-neutral presentation. The book is full of insight from the author's extensive knowledge and experience. Each lesson is designed to build upon previous lessons, but many can be read separately, thus providing the reader with a tutorial for routing basics as well as a means of learning about a particular routing concept quickly. Sams Teach Yourself Routing in 24 Hours presents routing fundamentals in twenty-four easy to follow lessons that build upon the previous lessons. Each of these lessons could stand alone as a tutorial on a particular routing concept thereby adding greater value to the book. In additional to these "easy to follow" lessons, this book is full of useful notes, tips, and cautions from the author's extensive experience that will prove invaluable to anyone who wants to learn routing as quickly as possible. The reader will also benefit from the exercises (each one based on an actual experience the author has encountered) at the end of each chapter.

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