Computer Networks

1
Computer Networks
2
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.

3
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

4
Internet Protocol (IPv4)

• 32-bit IP addresses
• Store internally as a single word
• Usually expressed to users 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 )

5
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.

6
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

7
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

8
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)

9
Some Well Known Port Numbers

• 23 (telnet)
• 21 (ftp)
• 13 (daytime)
• 25 (smtp (mail))
• 43 (whois)
• 144 (news)
• 80 (http web server)

10
Telnet Client

• telnet lthostnamegt ltport numbergt
• provides a general interface to request a service
  and return a text response.

11
Trace Route

• traceroute lthostnamegt (unix)
• tracert lthostnamegt (dos/windows)

12
Ping

• See if there is a connection between two machines
• See if a particular host is running
• ping lthostnamegt

13
Special Ip Addresses
14
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

15
Sample subnet for class B network
16
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.

17
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.

18
Network Routing
150.123.23.1
128.100.230.156
acavax.lynchburg.edu 161.115.147.1
128.173.133.102
161.115.100.22
128.100.200.165
128.100.201.155
161.115.144.2
161.115 lynchburg.edu
128.100 cns.vt.edu
208.22.66.1
128.173 cs.vt.edu
161.115.100.27
lasi-main
128.100.200.123
208.22.66.145
128.173.133.44
128.100.200.145
161.115.100.29
128.173.133.155
208.22.66 networkvirginia.net
GATEWAYS
208.22.66.100
208.22.66.102
208.22.66.101
19
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

20
Address Resolution Protocol (ARP)

• The source broadcasts the destination address to
  the local network (along with the local sources
  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

21
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.

22
Network Routing
150.123.23.1
128.100.230.156
acavax.lynchburg.edu 161.115.147.1
128.173.133.102
161.115.100.22
128.100.200.165
128.100.201.155
161.115.144.2
161.115 lynchburg.edu
128.100 cns.vt.edu
208.22.66.1
128.173 cs.vt.edu
161.115.100.27
lasi-main
128.100.200.123
208.22.66.145
128.173.133.44
128.100.200.145
161.115.100.29
128.173.133.155
208.22.66 networkvirginia.net
Routing Table for lasi-main 161.115 direct
delivery 208.22.66 direct delivery default
208.22.66.145
Routing Table for acavax 161.115 direct
delivery default 161.115.144.2
Routing Table for 208.22.66.145 208.22.66 direct
delivery 128.100 direct delivery 161.115
208.22.66.1 128.173 128.100.200.123 default
128.100.230.156
208.22.66.100
208.22.66.102
208.22.66.101