DNS (Domain Name System) is the system that translates an Internet name (that is easier for people to remember) to an IP address. A Domain Name could be for example http://www.networkstraining.com that the DNS translates to an IP address (74.54.219.242). If the address is registered in a reverse lookup zone, the IP address is translated back to the DNS name and it is called “reverse DNS lookup” or simply r-DNS.

A DNS system consists of two parts: a server and a resolver. The server keeps information about which DNS names correspond to which IP addresses. A Resolver is the software on the DNS client who is asking the server for information.

A DNS name consists of a host (e.g www means it is a webserver) and domain (e.g networkstraining.com). Domains are unique and are recorded in global databases.

ARP (Address Resolution Protocol) is a Layer2 protocol that is used to translate IP addresses (layer 3 in OSI model ) to MAC addresses (layer 2 in OSI model ), and is used when two hosts on a Local Area Ethernet Network want to communicate with each other. Of course ARP is used in other Layer 2 networks such as ATM for example, but the main usage of ARP is on Ethernet Networks.

The first time Host A wants to contact Host B, Host A sends an ARP Request (using a broadcast packet) on the LAN. The ARP packet basically asks the following “I have the IP address for Host B and I want to know the MAC address of Host B”.

Host B sees this broadcast packet and replies to Host A with an ARP Reply packet containing host B’s MAC address. Host A now saves this MAC address/IP address combination in its ARP cache and begins to communicate with host B. This communication happens with Layer2 MAC Addresses because the two hosts are on the same Layer 2 network.

To display the contents of a host’s ARP cache (table), use “arp-a” on a Windows platform, “arp” on a UNIX platform, and “show arp” on a Cisco IOS platform.

Just as in classical telephony, the voice signal is sent to the phone microphone handset in the form of an analog signal. An analog to digital converter transforms this signal into a digital one which is then encoded according to an audio format. There are different encoders for compression of a conversation with varying degrees of quality. Depending on the type of compression, some loss of voice signal information occurs, which, however, is mostly subjective and irrelevant.

Voice Data, after the compression, is transferred via the network. For this continuous data stream of compressed voice signal, the conversation is divided into small data packets, before they are sent to the network. Then, these packets arrive at the specific destination network “nodes”, so-called routers, which direct the IP-packets to their final point, maybe via different paths.

The voice packets, before they are sent to the network line, they are first temporarily stored in a memory location called the buffer. At the destination end, voice packets pass through a digital to analog converter so that to be transformed back to human speech. For example, the voice packets delivery can be compared with normal mailing post, which addresses, mails and delivers individual packets of data.

To transfer data using VoIP we currently use the so-called Internet protocol version 4 (IPv4). The next version – IPv6 – was specified in the mid 90-ies, but the need to expand its use has not been taken seriously until the last few years. Unfortunately, the IP protocol works on the principle of “Best Effort” and thus it does not guarantee 100% packet delivery. This is the main reason that we do not always have good quality in VoIP. The newest IP Protocol version 6 offers the so-called “quality of service” under which the voice data transmission will have better quality.

In order to establish a connection between a VoIP telephone system and the traditional PSTN telephone network, we need to use the so-called Gateways. These Gateways are connected to the IP Data network as well as with the PSTN telephone network and transmit requests in both directions. In this case, IP-packets from one side are converted into digital voice stream on the PSTN side.

Since everything now runs over IP (our computer data, voice, video etc) we can have the integration of different types of data on to a single IP network. This concept is usually called “network convergence”. The meaning of convergence is that we have one common network (the IP Network) which transmits all kinds of information – voice, data, video, text and images.

The Enhanced Interior Routing Protocol (EIGRP) is a hybrid routing protocol (it has characteristics of both distance vector and link state routing protocols) and is based on the Interior Gateway Routing Protocol (IGRP) which was its predecessor. Any Router which runs EIGRP will voluntarily resend its routing information to its EIGRP neighbors and also to its Interior Gateway Routing Protocol (IGRP) neighbors by changing its 32 bit EIGRP metric to a 24 bit metric for IGRP.

EIGRP is based on the DUAL (Diffusing Update Algorithm) routing algorithm which is the algorithm that works to guarantee that there are loop-free routing updates in the network and a fast convergence time is achieved. EIGRP collects routing data and stores it in three tables. Unlike many other distance vector routing protocols, EIGRP will not need any periodic route dumps in order to maintain the network topology table. The routing information is only exchanged when there is an establishment of new neighbor adjacencies and after this, routing updates will be sent only when there are routing changes in the network.

This routing protocol is Cisco proprietary. Distance vector routing protocols find the shortest path to a destination. What they do is exchange information about all the known destinations to their neighbors on the network. Information about the topology is never exchanged. Each node would then know about all the destinations present in the network and would be able to tell the distance to any destination through any of the neighbors. It does not need to know about the physical network topology though.

The Enhanced Interior Gateway Routing Protocol (EIGRP) is designed with certain enhancements over the classical distance vector protocols (like RIP and IGRP) so as to minimize the routing instability which usually occurs after changes in the topology and also to more efficiently utilize the processing power and bandwidth of the router.

As we said before, EIGRP uses three tables to store network information (neighbor table, topology table and routing table). The first is the neighbor table. This table has information about the neighbor routers which are directly connected.

Next it stores data in the topology table. The topology table will not, as the name suggests, store data about the entire network topology but rather it has the aggregated information about routing tables which are taken from all the directly linked neighbors.

The topology table will contain all the destination networks included in the network along with their metrics respectively. All destinations in the topology table will have a successor along with its feasible successor if they are available. The successor and feasible successor will serve as the next hop router for destination networks. Moreover, each destination can be shown as either Passive or Active. A destination is in the Passive state when the other routers know about a way to reach this destination. When a destination route is in the Active state it means that a topology change has just occurred for that particular destination network. The router now actively updates the routes for that particular destination.

There is also the routing table. In the routing table all the real routes to destinations are stored. This table is created by data coming out of the topology table and includes all the destinations and also the successor and feasible successor to those destinations.

To configure EIGRP on a router to include the 10.10.60.0 and 10.10.77.0 networks you would perform the follow commands (for example purposes only):

Router >enable
Router #config terminal
Router (config)# router eigrp 1
Router (config-router)#network 10.10.60.0
Router (config-router)#network 10.10.77.0
Router (config-router)#no auto-summary
Router (config-router)#end