IIIT’s Computer Network

Abhijeet Upadhyay ()

Abstract: -In order to gain a complete

understanding of the way a real time

network functions, it is best to actually

study a working network. The

motivation behind this task is that it

would give a more cogent picture of

what a network really is.

.

Introduction:- A network topology is the pattern of links connecting pairs of nodes of a network. A given node has one or more links to others, and the links can appear in a variety of different shapes. The simplest connection is a one-way link between two devices. A second return link can be added for two-way communication. Modern communications cables usually include more than one wire in order to facilitate this, although very simple bus-based networks have two-way communication on a single wire.

Network topology is determined only by the configuration of connections between nodes; it is therefore a part of graph theory. Distances between nodes, physical interconnections, transmission rates, and/or signal types are not a matter of network topology, although they may be affected by it in an actual physical network. Topologies are either physical or logical.

The various different types of topologies are:-

Linear Bus

A linear bus topology consists of a main run of cable with a terminator at each end (See fig. 1). All nodes (file server, workstations, and peripherals) are connected to the linear cable. Ethernet networks use a linear bus topology.


Fig. 1. Linear Bus topology

Advantages of a Linear Bus Topology

  • Easy to connect a computer or peripheral to a linear bus.
  • Requires less cable length than a star topology.

Disadvantages of a Linear Bus Topology

  • Entire network shuts down if there is a break in the main cable.
  • Terminators are required at both ends of the backbone cable.
  • Difficult to identify the problem if the entire network shuts down.
  • Not meant to be used as a stand-alone solution in a large building.

Ring

A layout scheme in which the network takes the form of a closed loop with the devices attached into the ring. Every workstation is linked to two others, one on each side. All data is passed from node to node in one direction only, each PC acting as a repeater for the next one in the loop. Response time is determined by the number of stations on the ring - the more there are, the slower it works.

Advantages of ring topology:

  • Growth of system has minimal impact on performance
  • All stations have equal access

Disadvantages of ring topology:

  • Most expensive topology
  • Failure of one computer may impact others
  • Complex

Star

A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub (See fig. 2).

Data on a star network passes through the hub before continuing to its destination. The hub manages and controls all functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.

Advantages of a Star Topology

  • Easy to install and wire.
  • No disruptions to the network then connecting or removing devices.
  • Easy to detect faults and to remove parts.

Disadvantages of a Star Topology

  • Requires more cable length than a linear topology.
  • If the hub, nodes attached are disabled.
  • More expensive than linear bus topologies because of the cost of the concentrators.

The protocols used with star configurations are usually Ethernet. Token Ring uses a similar topology, called the star-wired ring.

Tree

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable (See fig. 4). Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.

Advantages of a Tree Topology

  • Point-to-point wiring for individual segments.
  • Supported by several hardware and software venders.

Disadvantages of a Tree Topology

  • Overall length of each segment is limited by the type of cabling used.
  • If the backbone line breaks, the entire segment goes down.
  • More difficult to configure and wire than other topologies.

Mesh Topology

The topology of a network whose components are all connected directly to every other component.There are at least two nodes with two or more paths between them.

Advantages of Mesh Topology

  • Improves Fault Tolerance

Disadvantages of Mesh Topology

  • Expensive
  • Difficult to install
  • Difficult to manage
  • Difficult totroubleshoot

Computer networking devices are units that mediate data in a computer network. Computer networking devices are also called network equipment, Intermediate Systems (IS) or InterWorking Unit (IWU). Units which are the last receiver or generate data are called hosts or data terminal equipment.

List of computer networking devices

Common basic network devices:

  • Gateway: device sitting at a network node for interfacing with another network that uses different protocols. Works on OSI layers 4 to 7
  • Router: a specialized network device that determines the next network point to which to forward a data packet toward its destination. Unlike a gateway, it cannot interface different protocols. Works on OSI layer 3.
  • Bridge: a device that connects multiple network segments along the data link layer. Works on OSI layer 2.
  • Switch: a device that allocates traffic from one network segment to certain lines (intended destination(s)) which connect the segment to another network segment. So unlike a hub a switch splits the network traffic and sends it to different destinations rather than to all systems on the network. Works on OSI layer 2.
  • Ethernet hub: connects multiple Ethernet segments together making them act as a single segment. When using a hub, every attached device shares the same broadcast domain and the same collision domain. Therefore, only one computer connected to the hub is able to transmit at a time. Depending on the network topology, the hub provides a basic level 1 OSI model connection among the network objects (workstations, servers, etc). It provides bandwidth which is shared among all the objects, compared to switches, which provide a dedicated connection between individual nodes. Works on OSI layer 1.
  • Repeater: device to amplify or regenerate digital signals received while setting them from one part of a network into another. Works on OSI layer 1.
  • Amplifier:A device used to boost the strength of an electronic or optical signal, which is weakened (attenuated) as it passes through the transport network. Amplifiers add gain to the signal by an amount equal to the loss in the previous section of the network since last amplification.

Hardware or software components that typically sit on the connection point of different networks, e.g. between an internal network and an external network:

  • Proxy: computer network service which allows clients to make indirect network connections to other network services
  • Firewall: a piece of hardware or software put on the network to prevent some communications forbidden by the network policy.

A virtual LAN, commonly known as a VLAN, is a logically independent network. Several VLANs can co-exist on a single physical switch. IEEE 802.1Q is the predominant protocol.

Early VLANs were often configured to reduce the size of the collision domain in a large single Ethernet segment to improve performance. When Ethernet switches made this a non-issue (because they have no collision domain), attention turned to reducing the size of the broadcast domain at the MAC layer. It helps in centralized monitoring of network and also provides security for each vlan. Another purpose of a virtual network is to restrict access to network resources without regard to physical topology of the network, although the strength of this method is debatable.

Virtual LANs operate at layer 2 of the OSI model. However, a VLAN is often configured to map directly to an IP network, or subnet, which gives the appearance that layer 3 is involved.

Switch to switch links and switch to router links are called trunks. A router or Layer 3 switch serves as the backbone for traffic going across different VLANs.

VLANs can be configured in various ways;

  • Protocol level, IP etc
  • MAC address based.
  • IP subnet based.
  • Port based, and therefore real world based.

A Layer 2 device can implement VLANs in different ways;

  • Open VLANs have a single MAC address database for all VLANs.
  • Closed VLANs have a separate MAC address database for each VLAN.
  • Mixed Mode VLANs can be configured as Open or Closed on a VLAN basis.

Closed VLANs are generally considered more secure than Open VLANs.

Implementation of VLANs has increased the security between the two subnets.

Even though the Internet might be down, locally everyone can connect to others in the local network.

IIIT Network

  • Each sub-network forms a star topology. Each labs, hostels, etc. has a different switch to which all the pc in the respective rooms is connected to the corresponding switch. Overall it has a tree topology.
  • It has both wired and wireless LAN. Wired LAN connects all the hosts in the class rooms and the hosts in the faculty rooms. It also connects the ground floor of all the hostels and the faculty houses. The network forms the star topology.
  • Wireless LAN forms the part of all the campus including the hostels, main building.
  • The rate at which the data is transferred in wired LAN is normally between 5Mbps-10Mbps and for wireless LAN it is between 500kbps-1.5Mbps.

Connection to the Internet

  • There are two types of IP addresses, internal ip address and the external ip address. The 192.168.36.204 is the internal address and all the hosts in the LAN connect to the above server to access the internet. The 61.95.133.150 is the external address and is a class A address.
  • The ISP which in our case happens to be BEAM provides a 3Mbps bandwidth.
  • This 3Mbps speed is divided among all the hosts in the network.

Servers at IIIT

  • IIIT has nearly 10 servers, each performs a specific task i.e. one for proxy, one for DNS, one for mail server etc. Each server has its own public IP and each research center has its own external server.

Proxy Server

  • It caches the frequently used sites.
  • Internal IP of this is 192.168.36.204.
  • Whenever there is request for any site, it first checks in its buffer and if it is not there then only it sends a request for the site to ISP else it sends from its buffer itself. This saves the time and bandwidth.

Mail Server

  • Used as a mail server for the students. Everyone is provided with the login on this server.
  • Internal IP of this is 192.168.36.200.

Faculty Mail Server

  • Used as the mail server by the entire faculty.
  • Internal IP of this is 192.168.36.202.

Diglib

  • Used for storing all the new soft wares, online books, tutorials.
  • Internal IP of this is 192.168.36.150.

DNS Server

  • It stands for Domain Name Server. It has all the domains and their corresponding public ips.
  • Internal IP of theseare 192.168.36.210,192.168.36.204.

Research Mail Server

  • Used as a mail server for the MSBR students. Everyone is provide with the login on this server.
  • Internal IP on this is

192.168.36.250

ISP

  • We have 2 ISP , first one is of 3mbps and second one is of 200 kbps.
  • Currently only the first i.e 3 mbps one is being used.

IIIT wired network

  • The Optical Fiber Cable (OFC) connects IIIT with ISP. The cable which comes from ISP first passes through Media Converter (MC) and a Twisted Pair Cable (TPC) comes out.
  • This TPC is connected to the Layer 3 switch or a Router, which is the central device for whole of the IIIT network. The router is kept along with the servers in the server room.

Operation of Layer 3 Switch

  • It operates at the layer 3 of the OSI model which is the IP layer. Thus it understands only Ip addresses. This switch has many outlets on it where each outlet corresponds to a single subnet.
  • Each subnet is uniquely identified by its Gateway Ip address. Thus each subnet has its own default gateway.
  • The router has a buffer which keeps track of which subnet connected to which outlet, which helps the router to take the routing decisions.
  • Since a subnet contains many hosts while the outlet in the router is a single port, we use a Layer 2 switch which is connected to the router through the outlet and the hosts in the subnet are connected to the switch which contains many outlet ports.
  • Thus the switch connects to the router using a TPC.

VLAN in IIIT

  • In IIIT subnets are divided in such a way that all the 1st yr. labs come under one subnet, all the 3rd yr. labs come under one subnet, all the faculty hosts come under one subnet and so on.
  • Dividing hosts of similar weight into same subnet and of different into different subnets, implements VLAN.
  • By this it is impossible for a host on one subnet to know what is going on between two hosts of different subnet.
  • It also reduces the routing table, reduces the broadcast and structures the network well.

Operation of Layer 2 Switch

  • It understands only the Mac addresses of the hosts.
  • The switch has many ports on it where each host is connected to the port.
  • The switch has a buffer which keeps track of which Mac address of the host is connected to which port in the switch. This helps the switch to route packets in the same subnet or to the router.
  • System Administrator uses SNMP() to manage switches , each and every switch is given a separate IP , so that you can connect to a particular switch remotely and make necessary changes to the switch whenever needed. The IPs of switches are given as 172.16.30.* , name of switches is CISCO-2950 C.
  • Thus every host comes under one of the subnets. Each subnet has its own default gateway, subnet mask which are used by all the hosts in the subnet.
  • The MAC tables are formed dynamically .

For our lab: - Gateway is 172.16.14.1 Subnet mask is 255.255.255.192 IP Range is 172.16.14.1-63

For Faculty rooms and Classrooms: -Gateway is 192.168.36.1 , Subnet mask is 255.255.255.192 ,IP Range is 192.168.36.1-63

Connection Establishment i.e. how one host in the subnet connects to the other host on the same subnet, on other subnet.

Both hosts belong to the same host.

  • The source sends the packet to the switch to which it is connected.
  • Since it understands the Mac address, it gets the Mac address of the destination from the packet.
  • The switch searches in its buffer for the Mac address to get the port on which the destination sits and sends the packet through that port.
  • Thus the packet reaches the destination.

Both hosts belong to two different subnets.

  • The switch at the source after getting the packet from the source gets the Mac address of the destination.
  • The switch searches the Mac address in its buffer but will not find it and sends it to the Layer 3 switch.
  • The router now gets the Ip address of the destination from the packet and searches in its buffer for the corresponding port.
  • After getting the port the layer 3 switch sends the packet to the corresponding subnet.
  • The destination switch now searches its buffer for the Mac address of the port on which the destination sits and sends the packet to the destination.

Connecting to the Internet.

  • The Layer 3 switch sends the packet to the ISP keeping the source Ip as the public Ip of 192.168.36.204.
  • The proxy server keeps track of this request and the source of this request, so that when it gets the reply from the ISP for the above request it forwards the reply to the correct host.
  • Thus the connection is established.

WLAN

  • Wireless Local Area Network (WLAN) generally covers the whole organization.
  • WLAN uses IEEE 802.11 standards which are the most widely used. It uses Carrier Senses Multiple Access / Collision Avoidance.

Type of WLAN’s

  • 802.11a: This network is a 54Mbps network operating at 5 GHz frequency band that is less likely to be interfered with by commonly use items which generates waves.
  • 802.11b: This network is a 11Mbps operating at 2.4GHz frequency band, which might interfere with the radio waves generated by other devices operating at this frequency.
  • 802.11g: This network is a 54Mbps network operating at 2.4GHz frequency band.

Wireless network at IIIT is of 802.11g type.

WLAN Devices

  • Wireless Network Adapters (WNA) is devices that are generally attached to a PC. They send and receive signals from the nearest access points. WNA can link to another WNA or to AP. In the Ad-hoc mode WNA can link to another WNA. In the infrastructure mode WNA can like to AP.
  • Access Points (AP) are usually standalone boxes that attach or bridge the wireless network to the wired network via an Ethernet cable. AP is also available simply as an AP or can be combined with a hub, switch or a broadband route.
  • Initially WLAN didn’t have it’s own router , so the ARP traffic went to the router of wired LAN which slowed down both wired and wireless LAN. So to solve this problem they dedicated a router to WLAN.

In IIIT every classroom, every lab and every corridor in the hostels have Access Points and we connect to these access points using a wireless network adapter. The Access Points is turn is all connected to a gateway which in turn is connected to a layer 3 switch or router.

Security

802.11 wirelesses LAN provides Wired Equivalent Privacy (WEP) protocol to offer 3 main types of security services:-

Authentication

  • To verify user’s identity. IEEE 802.11 specifies two configurations for authentication.
  • The default setting is open system authentication in which anyone is allowed to join the network without any identity verification. It does not use any cryptographic tools for any security purposes.
  • The other setting is shared key authentication which uses RC4 as a cryptographic tool to authenticate user.

Confidentially