Download Overview/Questions Network Addresses Network Addresses

Overview/Questions
CS101 Lecture 19:
19:
Internetworking
± Review: what is a network?
± How do we tell the nodes on a network apart?
± What is an IP address, and what is its relation
to an Internet domain name?
± How do packets get where they need to go?
± What is the Internet, and how does the
Internet work?
Some diagrams copyright Jones and Bartlett
Some images courtesy Wikimedia Commons
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Network Addresses
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Network Addresses
Recall that a network is a collection of nodes.
Each node on a network must be uniquely identifiable.
A hostname is a mnemonic for an IP Address.
Address.
IP Address
An address made up of four oneone-byte integers, which
uniquely identifies a computer on the Internet.
For human consumption, IP addresses are specified in
dotteddotted-decimal form. Example: 128.197.26.3
Hostname
A name made up of words separated by dots which
uniquely identifies a computer on the Internet.
Hostnames for a given organization share some common
part called a domain name e.g. bu.edu for BU.
Is there a relationship between the parts of a hostname
and an IP address?
Examples: www.bu.edu,
www.bu.edu, smtp.bu.edu
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Network Addresses
Network Addresses
The IP Address is made up of 2 components:
the Network Number and the Host Number.
The IP Address space is divided into address classes,
which determines the size of the address space for
administrative purposes.
Class
Leading Bits Number of Networks Addresses/Network
Class A
0
128
16,777,214
Class B
10
16,384
65,534
Class C
110
2,097,152
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Figure 15.9
An IP address
is stored in
four bytes
Large telecom companies got Class A addresses.
Class A: first byte network number, and three bytes for host number.
Class B: first two bytes for network number, and two bytes for host number
Universities got Class B addresses.
Class C: first three bytes for network number, and one byte for host number
Smaller individual companies got Class C addresses.
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Issues in IP Addressing
Inefficient Use of Address Space
The original Internet Protocol uses 32 bits for each IP
Address. This makes it possible to have 232 or
4,294,967,296 unique IP addresses.
Problem: running out of IP addresses
± Most organizations too big for Class C addresses.
± A lot of Class A and B address space went unused.
± About 300 million of these are reserved for special purposes
Classless InterInter-Domain Routing (CIDR) permits
variable length network numbers (e.g. not just 8,
16, or 24 bits), which enables more efficient use
of the address space.
Example: 136.167.16.0/22
Interpret first 22 bits as ³network number.´
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CIDR helped a bit, but with the growth of PCs, network,
printers, wireless, etc., we are quickly running out of
available addresses.
The new IPv6 will alleviate this shortage.
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Internet Protocol version 6
Review: Internet Protocol
Recall that Internet Protocol is a way to
send data over a packet switching network
(network layer of OSI model).
IPv4:
IPv4: the original Internet Protocol uses 32 bits
(4 bytes) per address.
± 232 or 4,294,967,296 unique IP addresses.
± Messages are divided into packets,
packets, and each
packet is sent individually.
± Packets can take different routes from source
to destination.
IPv6:
IPv6: the new Internet Protocol uses 128 bits
(16 bytes) per address.
± This enables 2128 or about 3.403 × 1038 unique IP
addresses.
± IPv6 is the new standard on Windows Vista,
MacOSX,
MacOSX, and most Linux computers.
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Review: OSI Model
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IP Packet Header
At each layer, data is encapsulated in a
packet, along with protocol information.
Each protocol encapsulates some user data along
with a packet header.
header. The packet header contains
information used for routing and sequencing.
OSI Model.
This graphic is
taken from The
Abdus Salam
International
Centre for
Theoretical
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Physics.
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IP Packet Header
Routing
Routing
Routing directs the forwarding of IP packets from
their source to their destination.
A packet follows a route until it reaches its
destination or until times out (by too many hops).
The IP Packet Header is 20 bytes (160 bits):
Routing Tables
Lists of routes to various networks (by number).
The header specifies 13 fields, including protocol version,
version,
packet length,
length, source and destination IP addresses,
addresses,
fragment sequence,
sequence, and TTL (number of hops remaining).
± A network router does not know the exact location of
the destination, but rather which path should be
closer to it.
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Routing
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Internet Protocol Tools
Routing Algorithms
Used to determine the best route for a packet.
These tools are used by network administrators.
ping
A program used to test whether a particular
network computer is active and reachable.
Static Routing:
Routing: routes to networks are described
by fixed paths.
± does not compensate for changes in conditions
(congestion, host down, etc.).
traceroute
A program that shows the route a packet takes
across the Internet.
Adaptive Routing:
Routing: the system adapts the routes
dynamically to reflect changes in available routes.
± Open Shortest Path First (OSPF) protocol.
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Domain Name System
Domain Name System
Domain Name
The part of a hostname that specifies a specific
organization or group.
library.bu.edu
hostname
Domain Name System (DNS)
A distributed system for managing hostname
resolution ± the process of converting a domain
name to an IP address.
domain name
± Analogous to a ³phone book´ for Internet hosts.
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Domain Name System
TLD
TopTop-Level Domain (TLD)
The last section of a Domain Name that specifies
the type of organization or its country of origin.
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Domain Name System
Organizations based in countries other than the United
States use a toptop-level domain that corresponds to their
twotwo-letter country codes.
Figure 15.11
Some of the top-level domain names based on country codes
Figure 15.10 Top-level domains, including some relatively new ones
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Domain Name System
DNS Resolver
Domain Name Server
A computer that attempts to translate a hostname into an
IP address.
± In practice, a hostname resolution might require
queries to several DNS servers, each one with more
detailed information than the previous one.
Client programs such as web browsers send requests
to a DNS Resolver (in the operating system), which
communicates with the DNS servers.
The DNS Resolver typically maintains a cache to prevent
unnecessary queries to the DNS Servers.
DNS cache entries have a time to live (TTL) metric which
prevents stale information from being used.
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Domain Name System
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The Internet
DNS is updated by an administrator making
changes to one DNS server, which are then
propagated between servers.
The Internet is the interconnection of all networks,
connected together by routers/gateways.
± It takes several hours for changes to propagate
throughout the network.
By using packetpacket-switching technology,
technology, a common
addressing scheme, and open protocols,
protocols, messages
can be send from any node to any other node.
When DNS servers fail, users get a ³host not
found´ type of error.
Who owns the Internet?
± DNS update errors have caused large ³outages.´
± DNS servers are prime targets of denial of service
attacks.
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Summary
± Hostnames and IP address uniquely identify nodes on
a networks.
± Internet protocol enables transmission of packets.
± Routing directs packets from source to destination
across the network.
± DNS associates hostnames with IP addresses.
Combine these ingredients to get the Internet
(use only as directed; results may vary).
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