Presentation on theme: "NET0183 Networks and Communications Lectures 21 and 22 Support Protocols: DHCP and NAT 8/25/20091 NET0183 Networks and Communications by Dr Andy Brooks."— Presentation transcript:
8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks 2 The recommended textbook is Computer Networks and Internets by Douglas E. Comer http://www.coursesmart.com/0136066992/?a=1773944 www.pearson-books.com/student (for additional discounts and offers) http://www.coursesmart.com/0136066992/?a=1773944 www.pearson-books.com/student
Cisco IOS Wikipedia 6. mars 2010 4 “Cisco IOS (originally Internetwork Operating System) is the software used on the vast majority of Cisco Systems routers and current Cisco network switches. (Earlier switches ran CatOS). IOS is a package of routing, switching, internetworking and telecommunications functions tightly integrated with a multitasking operating system. The first IOS was written by William Yeager.Cisco Systemsroutersnetwork switchesCatOSmultitaskingWilliam Yeager Cisco IOS has a characteristic command line interface (CLI), whose style has been widely copied by other networking products.”command line interface NET0183 Networks and Communications by Dr Andy Brooks 8/25/2009
Cisco IOS DHCP Server http://www.cisco.com/en/US/docs/ios/12_0t/12_0t1/feature/guide/Easyip2.html#wp4363 7 NET0183 Networks and Communications by Dr Andy Brooks 8/25/2009 “Dynamic Host Control Protocol (DHCP) enables you to automatically assign reusable IP addresses to DHCP clients. The Cisco IOS DHCP Server feature is a full DHCP server implementation that assigns and manages IP addresses from specified address pools within the router to DHCP clients. If the Cisco IOS DHCP Server cannot satisfy a DHCP request from its own database, it can forward the request to one or more secondary DHCP servers defined by the network administrator. Figure 1Figure 1 shows the basic steps that occur when a DHCP client requests an IP address from a DHCP server. The client, Host A, sends a DHCPDISCOVER broadcast message to locate a Cisco IOS DHCP Server. A DHCP server offers configuration parameters (such as an IP address, a MAC address, a domain name, and a lease for the IP address) to the client in a DHCPOFFER unicast message.”
Cisco IOS DHCP Server http://www.cisco.com/en/US/docs/ios/12_0t/12_0t1/feature/guide/Easyip2.html#wp4363 8 NET0183 Networks and Communications by Dr Andy Brooks 8/25/2009 Figure 1 DHCP Request for an IP Address from a DHCP Server The DHCPREQUEST is broadcast so that all DHCP servers know which offer the client has accepted. ( A client can receive DHCP offers from multiple DHCP servers.)
Cisco IOS DHCP Server http://www.cisco.com/en/US/docs/ios/12_0t/12_0t1/feature/guide/Easyip2.html#wp4363 9 NET0183 Networks and Communications by Dr Andy Brooks 8/25/2009 “A DHCP client may receive offers from multiple DHCP servers and can accept any one of the offers; however, the client usually accepts the first offer it receives. Additionally, the offer from the DHCP server is not a guarantee that the IP address will be allocated to the client; however, the server usually reserves the address until the client has had a chance to formally request the address.” “The client returns a formal request for the offered IP address to the DHCP server in a DHCPREQUEST broadcast message. The DHCP server confirms that the IP address has been allocated to the client by returning a DHCPACK unicast message to the client.” acknowledgement/staðfesting
Cisco IOS DHCP Server http://www.cisco.com/en/US/docs/ios/12_0t/12_0t1/feature/guide/Easyip2.html#wp4363 10 NET0183 Networks and Communications by Dr Andy Brooks 8/25/2009 “The formal request for the offered IP address (the DHCPREQUEST message) that is sent by the client is broadcast so that all other DHCP servers that received the DHCPDISCOVER broadcast message from the client can reclaim the IP addresses that they offered to the client.” “If the configuration parameters sent to the client in the DHCPOFFER unicast message by the DHCP server are invalid (a misconfiguration error exists), the client returns a DHCPDECLINE broadcast message to the DHCP server.” “The DHCP server will send to the client a DHCPNAK denial broadcast message, which means the offered configuration parameters have not been assigned, if an error has occurred during the negotiation of the parameters or the client has been slow in responding to the DHCPOFFER message (the DHCP server assigned the parameters to another client) of the DHCP server.” A NAK is a negative acknowledgment from DHCP.
12 NET0183 Networks and Communications by Dr Andy Brooks 8/25/2009 End-users whose computers coordinate with a DHCP server to obtain an IP address normally do not need to worry about their IP address expiring. Note that a client can ask a DHCP server to allocate the previously allocated IP address.
NAT http://foldoc.org/nat 21 Network Address Translation A technique in which a router or firewall rewrites the source and/or destination Internet addresses in a packet as it passes through, typically to allow multiple hosts to connect to the Internet via a single external IP address. NAT keeps track of outbound connections and distributes incoming packets to the correct machine.routerfirewallInternet addresseshostsInternetIP address NET0183 Networks and Communications by Dr Andy Brooks 8/25/2009
port numbers Transport Layer protocols such as TCP and UDP specify a source and destination port number in their packet headers. – Port numbers are an abstract set of numbers independent of an operating system. Operating systems use process identifiers, job names, or task identifiers to refer to processes. A port number is a 16-bit unsigned integer (0 to 65535). A process associates with a particular port to send and receive data. – The process will listen for incoming packets whose destination port number and IP destination address match that port. – The process will send out packets whose source port number is set to that port. 31 NET0183 Networks and Communications by Dr Andy Brooks 8/25/2009
NET0183 Networks and Communications by Dr Andy Brooks 32 Packet delivered to an abstract port number is delivered to the correct process.
33 http://skogberg.eu/ia/img/protocolStack.png Two important protocols in the transport layer are TCP and UDP.
proxy server http://en.wikipedia.org/wiki/Proxy_server 7.3.2010 http://en.wikipedia.org/wiki/Proxy_server 37 In computer networks, a proxy server is a server (a computer system or an application program) that acts as an intermediary for requests from clients seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a different server. The proxy server evaluates the request according to its filtering rules. For example, it may filter traffic by IP address or protocol. If the request is validated by the filter, the proxy provides the resource by connecting to the relevant server and requesting the service on behalf of the client. A proxy server may optionally alter the client's request or the server's response, and sometimes it may serve the request without contacting the specified server. In this case, it 'caches' responses from the remote server, and returns subsequent requests for the same content directly.computer networksserverclientsIP addressprotocolcaches
proxy server http://en.wikipedia.org/wiki/Proxy_server 7.3.2010 http://en.wikipedia.org/wiki/Proxy_server to keep machines behind it anonymous (mainly for security) to speed up access to resources (using caching) – web proxies are commonly used to cache web pages from a web server to apply access policy to network services or content – e.g. to block undesired sites to log usage i.e. to provide company employee Internet usage reporting 8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks 38 A proxy server has many potential purposes, including: