Computer Networks

IP Addresses• Before we communicate with a computer

on the network we have to be able to identify it.

• Every computer on a network must have a unique “address”.

• The system used on the Internet is called Internet Protocol (IP). Every machine on the Internet has a unique IP address.*

Internet Protocol (IP)

• Internet protocol has been revised– IP version 4 (IPv4) is the most widely used. – IP version 6 (IPv6) will eventually replace it.

• IPv4 uses 32-bit host addresses

• IPv6 uses 128-bit host addresses

Internet Protocol (IPv4)

• 32-bit IP addresses– Store internally as a single word– Usually expressed to user’s using “dotted

decimal notation” (e.g., 161.115.147.1) where each of the four numbers range from 0 to 255.

– Usually aliased to host “domain name” of the system (e.g. hobb-0113-Instrl.lynchburg.edu, www.google.com, www.amazon.com )

Domain Name System (DNS)

• How can we map host names to IP addresses?

• Domain Name Servers provide names in response to client requests.

• A hierarchical representation provides name management at each level of domain name.

Domain Name Management

• Each level is responsible for assigning its own names– .edu is the top-level domain for educational

institutions in the US. We need to get permission from the managers of the .edu domain before we can use lynchburg.edu

– lynchburg.edu is the Lynchburg College domain name. We can name our computers anything we want, (e.g., altair.lynchburg.edu).

– If we wanted to we could create a cs.lynchburg.edu subdomain

Client/Server Models

• Network interaction is based on an asymmetrical relationship– Server

• provides services to authorized clients

• typically handles many clients simultaneously

• works passively, driven by client requests

– Client• requests services

• typically initiates

Port Numbers

• IP addresses identify host machines only.• Port numbers identify services on machines.

– 16-bit port numbers provide values (0-65535)• “Well-known ports” (0-1023)• Registered ports (1024-49151) (49152 is 75% of available port

numbers)• Dynamic or private ports

• Internet Assigned Number Authority (IANA)– Determines (0-1023)– Registers (1024-49151)– Does not control (49151-65535)

Some Well Known Port Numbers

• 23 (telnet)

• 21 (ftp)

• 13 (daytime)

• 25 (smtp (mail))

• 43 (whois)

• 144 (news)

• 80 (http web server)

Telnet Client

• telnet <hostname> <port number>– provides a general interface to request a service

and return a text response.

Trace Route

• traceroute <hostname> (unix)

• tracert <hostname> (dos/windows)

Ping

• See if there is a connection between two machines

• See if a particular host is running

ping <hostname>

Special Ip Addresses

Subnets• Problem:

– We are rapidly running out of IP addresses– Many networks do not have the maximum number

of hosts that can be supported

• Temporary Solution:– Create subnet. Subnet works with smaller numbers

of hosts.

• Changes the distribution of network and host addresses– Uses some of the bits originally allocated for the

host address for the network address

Sample subnet for class “B” network

Types of Routing

• Direct Routing– Both machines are on the same network

– Machines communicate directly, e.g., on the same Ethernet

• Indirect Routing– Machines are on different physical networks

– There is no direct connection

– Datagrams travel from source to destination via one or more gateways or routers.

• Hosts can determine which method to use by comparing their network address with the network address of the datagram.

Gateways and Routers

• A gateway is a host that belongs to more than one network

• A gateway has more than one IP address, one for each network that includes it.

• Gateways can also work as hosts, but more frequently gateways provide routing as their principal function.

• The terms router and gateway are often used interchangeably, but the term router is more often used to describe custom hardware dedicated to routing.

Network Routing

161.115lynchburg.edu

161.115.100.22acavax.lynchburg.edu161.115.147.1

161.115.100.27

161.115.100.29

208.22.66networkvirginia.net

128.100.200.123

128.173cs.vt.edu

128.100cns.vt.edu

128.173.133.44

161.115.144.2

128.100.200.145

208.22.66.1

208.22.66.145

208.22.66.100

208.22.66.101

208.22.66.102

128.100.200.165128.100.201.155

128.100.230.156

128.173.133.155

128.173.133.102

lasi-main

150.123.23.1

GATEWAYS

Routing on a LAN

• Datagrams can be routed directly to a destination host that is connected to the same LAN

• Ethernet cards all have a unique 6-byte address (e.g., 08-f3-32-a3-b5-23)

• The hosts maintain a table of IP to Ethernet address translations

• This table is maintained automatically using ARP

Address Resolution Protocol (ARP)

• The source broadcasts the destination address to the local network (along with the local source’s address) requesting the Ethernet address of the corresponding host

• The destination host responds by sending its Ethernet address to the source host

• The destination host sends the data to the destination and records the IP/Ethernet address pair in a cache for future use.

• Cache entries time-out and must be reacquired

Basics of Internet Routing

• Look at the destination IP address

• Extract the network address

• If network is directly connected, send the datagram directly. Use ARP to resolve the physical address if necessary.

• If network is indirectly connected, lookup the address of the network in a routing table.

Network Routing

161.115lynchburg.edu

161.115.100.22acavax.lynchburg.edu161.115.147.1

161.115.100.27

161.115.100.29

208.22.66networkvirginia.net

128.100.200.123

128.173cs.vt.edu

128.100cns.vt.edu

128.173.133.44

161.115.144.2

128.100.200.145

208.22.66.1

208.22.66.145

208.22.66.100

208.22.66.101

208.22.66.102

128.100.200.165128.100.201.155

128.100.230.156

128.173.133.155

128.173.133.102

Routing Table for acavax161.115 direct deliverydefault 161.115.144.2

Routing Table for lasi-main161.115 direct delivery208.22.66 direct deliverydefault 208.22.66.145

lasi-main

Routing Table for 208.22.66.145208.22.66 direct delivery128.100 direct delivery161.115 208.22.66.1128.173 128.100.200.123default 128.100.230.156

150.123.23.1