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Transcript
Exam 3 Review Part 1: long distance digital connection Central offices are owned by telephone companies Phone switches are not routers or WAN switches Be clear about the differences between analog and digital long-distance circuits A leased line is a T1, T2, or T3 digital long-distance circuit To connect a computer network to a leased line, DSU/CSU is needed to translate between network industry formats and telephone industry formats Part 2: WAN technologies Compared with building a LAN, the new challenges are that (a) the network is now much larger; (b) long-distance circuits are needed; (c) there are possibly multiple routes from one host to another host When the network is much larger, using MAC address based forwarding tables will make the network so slow because the forwarding table will be so large. To solve this problem, hierarchical addresses must be used The unique advantage of hierarchical addresses is that they allow people to merge several rows (or records) in the forwarding table into a single row; as a result, the number of records within a routing table will be determined by the number of external divisions and the number of local LAN segments To tackle challenge (b), a leased line could be used When hierarchical addresses are used, a forwarding table is called a routing table To tackle challenge (c), we should do shortest path routing; the results of shortest path routing must be recorded in the routing table, since the processor of every router will use the routing table directly when they determine where to forward a packet Different routers have different routing tables; The shortest path in terms of router 1 may NO LONGER be the shortest path in terms of router 2 Part 3: protocol and layering To make large networks quick, we must use hierarchical addresses. However, NIC cards can only use MAC addresses to do address filtering, so layering is necessary A key idea of layering is to add two headers to a single packet The information contained in the routing layer header is used to do routing; the information contained in the NIC layer header is to survive address filtering “Nested headers” means that the routing layer header is part of the NIC layer payload Because different NIC cards will use different MAC addresses to do address filtering, the packet must change its NIC layer headers during the transmission from the sender host to the receiver host The same layer will have different names in the OSI reference model, the TCP/IP reference model, and the 220 reference model Part 4: Internetworks and IP addresses When multiple organizations interconnect their LANs or WANs together, we will get an Internetwork The internet is the largest internetwork; it includes a set of routers which do not belong to any organizations: they form the backbone of the internet IP addresses can guarantee that every host in an internetwork can have a unique IP address Because IP addresses are hierarchical addresses, an IP address must be split into two parts: the prefix and the suffix Without the prefix, we do not know which physical network the host belongs to Without the suffix, we could not distinguish two hosts within the same physical networks All the possible IP addresses are broken down into 4 classes to make both big organizations and small organizations happy Class A networks are the largest; Class C networks are the smallest Each class has different number of bits in the prefix and the suffix The class of an IP address is uniquely determined by the first 4 bits of the IP address Part 5: IP packet forwarding A router may have multiple IP addresses belonging to different physical networks IP addresses are useless to Bridges An IP packet has two headers: the routing layer header and the NIC layer header All routing tables use network addresses in the destination address column No host in a physical network can be assigned the broadcast address or the network address Shortest path routing is still necessary To match a packet against a routing table, specific IP masks are necessary; otherwise, a match will never be found and every packet will be discarded by the routers