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Exam 3 Review
Part 1: long distance digital connection
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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
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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
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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
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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
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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