Lecture#1 on Lecture#1 on InternetInternet

Internet AddressingInternet Addressing

IP address: pattern of 32 or 128 bits often IP address: pattern of 32 or 128 bits often represented in dotted decimal notationrepresented in dotted decimal notation

Mnemonic address:Mnemonic address: Domain namesDomain names Top-Level DomainsTop-Level Domains

Domain name system (DNS)Domain name system (DNS) Name serversName servers DNS lookupDNS lookup

Internet Corporation for Internet Corporation for Assigned Names & Assigned Names & Numbers (ICANN)Numbers (ICANN)

Allocates IP addresses to ISPs who Allocates IP addresses to ISPs who then assign those addresses within then assign those addresses within their regions.their regions.

Oversees the registration of domains Oversees the registration of domains and domain names.and domain names.

Proxy ServerProxy Server

A A server that sits between a that sits between a client application, such as a Web , such as a Web browser, and a real server. It browser, and a real server. It intercepts all requests to the real intercepts all requests to the real server to see if it can fulfill the server to see if it can fulfill the requests itself. If not, it forwards the requests itself. If not, it forwards the request to the real server.request to the real server.

Two PurposeTwo Purpose Improve PerformanceImprove Performance:: Proxy servers can Proxy servers can

dramatically improve performance for groups of dramatically improve performance for groups of users. This is because it saves the results of all users. This is because it saves the results of all requests for a certain amount of time. Consider the requests for a certain amount of time. Consider the case where both user X and user Y access the World case where both user X and user Y access the World Wide Web through a proxy server. First user X Wide Web through a proxy server. First user X requests a certain Web page, which we'll call Page requests a certain Web page, which we'll call Page 1. Sometime later, user Y requests the same page. 1. Sometime later, user Y requests the same page. Instead of forwarding the request to the Web server Instead of forwarding the request to the Web server where Page 1 resides, which can be a time-where Page 1 resides, which can be a time-consuming operation, the proxy server simply consuming operation, the proxy server simply returns the Page 1 that it already fetched for user returns the Page 1 that it already fetched for user X. Since the proxy server is often on the same X. Since the proxy server is often on the same network as the user, this is a much faster operation. network as the user, this is a much faster operation.

Two PurposeTwo Purpose

Filter Requests:Filter Requests: Proxy servers can Proxy servers can also be used to filter requests. For also be used to filter requests. For example, a company might use a example, a company might use a proxy server to prevent its proxy server to prevent its employees from accessing a specific employees from accessing a specific set of Web sites. set of Web sites.

Distributed NetworksDistributed Networks the design of the ARPANet was influenced by the ideas of Paul Baran, a the design of the ARPANet was influenced by the ideas of Paul Baran, a

researcher at the RAND Instituteresearcher at the RAND Institute Baran proposed 2 key ideas: Baran proposed 2 key ideas: distributed networkdistributed network and and packet-switchingpacket-switching

recall: the ARPANet was funded by the Dept of Defense for communicationsrecall: the ARPANet was funded by the Dept of Defense for communications as such, it needed to be resistant to attack or mechanical failureas such, it needed to be resistant to attack or mechanical failure

Packet SwitchingPacket Switching in a packet-switching network, messages to be sent over the in a packet-switching network, messages to be sent over the

network are first broken into small pieces known as network are first broken into small pieces known as packetspackets these packets are sent independently to their final destinationthese packets are sent independently to their final destination

Advantages of PacketsAdvantages of Packets1.1. sending information in smaller units increases the efficient use of sending information in smaller units increases the efficient use of

connectionsconnections large messages can't monopolize the connectionlarge messages can't monopolize the connection analogy: limiting call lengths at a pay phone to limit waitinganalogy: limiting call lengths at a pay phone to limit waiting

2.2. transmitting packets independently allows the network to react to transmitting packets independently allows the network to react to failures or network congestionfailures or network congestion routers (special-purpose computers that direct the flow of messages) routers (special-purpose computers that direct the flow of messages)

can recognize failures or congestion and reroute the packet around can recognize failures or congestion and reroute the packet around trouble areastrouble areas

3.3. breaking the message into packets can improve reliabilitybreaking the message into packets can improve reliability since the packets are transmitted independently, it is likely that at least since the packets are transmitted independently, it is likely that at least

part of the message will arrive (even if some failures occur within the part of the message will arrive (even if some failures occur within the network)network)

software at the destination can recognize which packets are missing software at the destination can recognize which packets are missing and request retransmissionand request retransmission

Protocols and AddressesProtocols and Addresses

the Internet allows different types of computers from around the world to the Internet allows different types of computers from around the world to communicatecommunicate this is possible because the computing community agreed upon common this is possible because the computing community agreed upon common

protocolsprotocols (sets of rules that describe how communication takes place) (sets of rules that describe how communication takes place) the two central protocols that control Internet communication are:the two central protocols that control Internet communication are:

1.1. Transmission Control Protocol Transmission Control Protocol ((TCPTCP) ) 2.2. Internet Protocol Internet Protocol ((IPIP))

these protocols rely on each computer having a unique these protocols rely on each computer having a unique identifier (known as an identifier (known as an IP addressIP address))

analogy: street address + zip code provide unique address analogy: street address + zip code provide unique address for your house/dormfor your house/dorm

using this address, anyone in the world can send you using this address, anyone in the world can send you a lettera letter

an IP address is a number, written as a dotted sequence such an IP address is a number, written as a dotted sequence such as 147.134.2.84as 147.134.2.84

each computer is assigned an IP address by its Internet each computer is assigned an IP address by its Internet Service Provider (ISP) Service Provider (ISP)

some ISPs (e.g., AOL, most colleges) maintain a pool of IP some ISPs (e.g., AOL, most colleges) maintain a pool of IP addresses and assign them dynamically to computers each addresses and assign them dynamically to computers each time they connecttime they connect

TCP/IPTCP/IP Transmission Control Protocol (TCP) Transmission Control Protocol (TCP)

controls the method by which messages are broken down into packets and then controls the method by which messages are broken down into packets and then reassembled when they reach their final destinationreassembled when they reach their final destination

Internet Protocol (IP) Internet Protocol (IP) concerned with labeling the packets for delivery and controlling the packets’ paths concerned with labeling the packets for delivery and controlling the packets’ paths

from sender to recipientfrom sender to recipient

Routers and DNSRouters and DNS the Internet relies on special purpose computers in the networkthe Internet relies on special purpose computers in the network

routersrouters are computers that receive packets, access the routing information, and pass are computers that receive packets, access the routing information, and pass the packets on toward their destinationthe packets on toward their destination

domain name servers domain name servers are computers that store mappings between domain names and are computers that store mappings between domain names and IP addressesIP addresses

domain namesdomain names are hierarchical names for computers (e.g., bluejay.creighton.edu) are hierarchical names for computers (e.g., bluejay.creighton.edu)

they are much easier to remember and type than IP addressesthey are much easier to remember and type than IP addresses domain name servers translate the names into their corresponding IP addressesdomain name servers translate the names into their corresponding IP addresses