Routers and Networking

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Ch.7 - Routers and Networking 1 Router development 2 IP addresses 3 Subnet mask 4 Routing decision 5 Routing table 6 Routing protocols 7 Example - RIP 8 Redundancy 9 Dynamic update

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Ch.7 - Routers and Networking

IP networks are packet-switched networks, which means that the packets can take different routes between a source and a destination host. The routing functions are performed by a router, which is a computer with a single task: to transfer data packets between different physical networks. The whole Internet consists of a number of networks connected to each other via routers. The first routers appeared in the middle of the 80´s. At that time the router function was performed by UNIX workstations. The UNIX workstations had two or more network interface cards and router software installed. Companies such as Cisco, Wellfleet and Proteon were born and developed router technology. Later the router became a dedicated device with a single function of routing packets. In the mid 90’s almost all networks were router based, and new routing techniques that supported higher bandwidths were developed. Today, new applications such as multimedia demand smaller delays from routers. To satisfy this demand, new types of fast routing technologies, called level 3 switching, are being developed.

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Ch.7 - Routers and Networking

The addresses used by the IP protocol are 32-bit numbers. Every host and physical network must be assigned a unique 32-bit address number. If you are running a local TCP/IP network that does not have connections with other public networks such as the Internet, you may assign these numbers according to your personal preferences. However, for sites on the Internet, numbers are assigned by a central authority, such as INTERNIC in the USA or by the local service providers. IP addresses are split up into four 8-bit numbers called octets. For example, 193.12.15.1. This format is often referred to as the dotted quad notation. Since every octet consists of 8 bits, the maximum decimal number for an octet is 255. A company usually gets a number of consecutive IP-numbers from an Internet Service Provider. Those numbers must be used for the network itself, for the connected routers and hosts. Quad notation is useful because IP addresses are split into a network number and local numbers. The network number is contained in the leading octets, while the local numbers are contained in the remainder.

In the picture for example you can see two different networks. The first networks is identified by the IP-number 193.12.15.0 and the other network is identified by the IPnumber 10.5.17.0. On every network there are two hosts. Observe that the first three octets in the IP-numbers are the same as in the IP-number of the networks.The router that connects the two networks has two interfaces with IP-numbers that belong to each respective network.

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Ch.7 - Routers and Networking

The number of IP addresses assigned depends on the size of your network. To accommodate different needs, there are several classes of IP addresses. Those classes are called A, B , C or D.

There are 128 class A networks. You can have up to 16 million different hosts in every class A network. As you can very well understand, there are very few owners that have class A networks. Some owners are IBM, Japan and General Electric. If the first octet in an IPaddress has a number from 0 to 127 it is a class A network. There are about 16 000 class B networks. You can have up to 65 000 different hosts in every class B network. Some owners of class B networks are big companies. For instance, Ericsson has several class B networks. You can recognise a B network from its first octet. It has a value from 128 to 191. There are about 2 million class C networks. You can have up to 255 different hosts in every class C network. Smaller companies are typical owners of class C networks. You can recognise a C network from its first octet. It has a value from 192 to 223. D networks are used for a special purpose called multicasting. Multicasting is a data traffic that is sent to a group of destinations, for example videoconferences and radio broadcasts over the Internet. You can recognise a multicast address from its first octet. It has a value from 224 to 239.

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Ch.7 - Routers and Networking

The subnet mask is used to tell what part of the IP address is used for the network and which for local addresses. The subnet mask assigns a binary 1 for the bits that belong to the network part while it assigns a binary 0 for the bits that belong to the local address part. In the picture you can see the default subnet masks for the A, B and C classes. The bits that belong to the network part are locked, while the bits that belong the local addresses can be changed by the owner. This is used if the owner wants to divide his network into several different subnets.

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Ch.7 - Routers and Networking

The router algorithm includes four different steps. 1. The first question is if the destination host is on a network that is directly connected to the router. If the answer to this question is yes the router makes a direct delivery to the destination host. 2. Otherwise the next question is if the destination host is to be found in the routing table. If the answer is yes, then the router sends the packets to a router that is closer to the destination host. 3. Otherwise the router looks to see if the network, to which the destination host is connected, is to be found in the routing table. If the answer is yes, then the router sends the packets to a router that is closer to the destination host. 4. The last chance for the router is to use a default router to send the packet to. Now it is up to the default router to find the destination host. The default router is sometimes also called the default gateway.

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Ch.7 - Routers and Networking

How does the router choose where to send the packets? The router keeps the routing information in a table. This information tells the router how to reach remote networks and hosts. A catch-all entry, also called the default router must generally be supplied. All packets to an unknown network or host are sent to the default router. In the picture you see a router with its specific routing table. If the packets are to be delivered to the host with IP-number 192.1.5.10, the router sees that the destination host belongs to the network 192.1.5.0. The routing table says that this network can be reached via the router 192.1.8.2.

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Ch.7 - Routers and Networking

Routing protocols are used so that the routers in a network can learn things from other routers automatically. If you have a Point-to-Point network with no alternative routes, then there is no need for routing protocols. In this case routing protocols just give you a lot of ”overhead” traffic in your network. But if you have many different routes then there is good reason to use routing protocols. For example, if one link goes down, the routing protocol can teach other routers in the network a new way to the destination. There are two groups of routing protocols: The first group is called Interior Gateway Protocol, IGP which is used inside a local network. This network is also called an autonomous system or AS. Three examples of IGP protocols are: 1. RIP, which stands for Routing Information Protocol. 2. IGRP, which stands for Interior Gateway Routing Protocol and EIGRP, which stands for Enhanced Interior Gateway Routing Protocol and they are two Cisco specific protocols. 3. OSPF, which stands for Open Shortest Path First.

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Ch.7 - Routers and Networking

RIP and OSPF are implemented by every vendor and are commonly used in mixed environments. The 2nd group of protocols is called Exterior Gateway Protocols, EGP, and is used between autonomous systems. EGP, is normally used by the Internet service providers. Two examples of EGP routing protocols are: 1. BGP, which stands for Border Gateway Protocol. 2. GGP, which stands for Gateway to Gateway protocol. This protocol is history and is not used today.

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Ch.7 - Routers and Networking

Routing Information Protocol, or RIP, is implemented by all vendors of routing equipment. RIP is a quite simple routing protocol. It only takes into consideration the number of routers you need to pass in order to reach the destination host. This is called the cost to reach the destination host. The RIP does not take into account the quality or bandwidths of the connected links. RIP broadcasts its routing table every 30 seconds in order to share with other routers. This gives a lot of overhead traffic on the network. This picture shows three routers using RIP. The router B, for example was originally configured with the information about the directly connected networks. The rest of the information in the routing table has been learned from the other routers in the network.

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Ch.7 - Routers and Networking

Redundancy means having more than one possible path to reach the destination host. In the picture you can see that there are two alternative ways between the host A and host B which gives us redundancy. On the Internet for example, there are many alternative ways to reach a host or a network, which means that Internet has a lot of redundancy.

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Ch.7 - Routers and Networking

To use all the benefits of a secure and redundant network structure a routing protocol is needed. The routing protocol must be used so that the routers can exchange information continuously about the status of the links between the different routers. This means that the router information is dynamically updated. In the picture you can see that a path between routers has been broken. With the help of routing protocols the information about the broken link is sent to other routers in the network. This means that the routers will send the packets using an alternative path.

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