Download Computer Networks - Lynchburg College

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