INTERNETWORKING I

CHAPTER 11 STUDY GUIDE

What devices use Layer 2 addresses to segment networks?

switches, bridges

What device uses Layer 3 addresses to segment networks?

routers

How does a router recognize one network from another?

by the IP number. Each network has a unique IP address.

What is an interface on a router? How is each one identified?

a port. Each interface must have a unique network or subnet address.

What is ARP? What does it do?

Address Resolution Protocol. If a sending device knows the IP address of the destination host but not the MAC address, then ARP finds the MAC address that matches the IP address. It then sends this information back to the device that needed the information.

How does a router keep track of MAC addresses?

It builds an ARP table that resolves MAC address to IP addresses. ARP tables are also maintained by other devices on the network, too, including hosts.

What is the structure of an ARP request?

Three parts:

MAC header – has the destination’s MAC address designated as FF-FF-FF-FF-FF-FF (broadcast) and the source’s MAC address (which it knows)

IP header – has the destination host IP address and the source host IP address specified

ARP Request Message – asks for the MAC address of the destination

This message goes out to all devices on the network. The device whose IP address matches the IP header portion of the ARP request, sends back its MAC address to the sending host (ARP reply). This host then resolves the MAC address with the IP address, puts it in its ARP table, and correctly addresses the packet and sends it on its way.

What is RARP? What does it do?

Reverse Address Resolution Protocol. This is the opposite of ARP. In ARP, the sending device has the destination’s IP, but not its MAC. With RARP, the sending device has the destination’s MAC not its IP address. So RARP resolves MAC addresses with known IP addresses. This information goes into the ARP table of the device.

What is an ARP table?

Routers and other devices on a network build these tables. They map MAC addresses to IP addresses. They run in RAM of the device. In routers, ARP tables can contain MAC and IP addresses of devices on other networks, as well as on their own.

What is the protocol used that will dynamically assign IP numbers to devices on a network?

DHCP (Dynamic host configuration protocol). It allows a host to obtain an IP address quickly and dynamically.

What is a default gateway?

The IP address of the router interface that connects to the network segment that the source host is located on.

What is proxy ARP?

A variation of ARP. It uses an intermediate device (such as a router) to send an ARP reply on behalf of the destination host to the source host. A router running proxy ARP responds to ARP replies with its own (the router’s) MAC address if the ARP request is for a destination host that isn’t on the router’s local subnet.

Place an “x” in the appropriate column of the following table to identify each protocol:

Protocol	Routed	Routing
Appletalk	x
RIP		x
OSPF		x
IPX	x
EIGRP		x
SPX	x
IGRP		x
IP	x

What is the difference between a routed protocol and a routing protocol (i.e., what is each used for)?

A routed protocol is used to route users’ messages (i.e., between devices on a LAN). A routing protocol is used between routers to send path information.

Place an “x” in the appropriate column of the following table to identify each type of routing protocol:

Protocol	Distance-Vector	Link-State	Interior Gateway	Exterior Gateway	Proprietary	Dynamic Routing	Static Routing
RIP	X		X			X
IGRP	X		X		X	X
EIGRP	X	X	X		X	X
OSPF		X	X			X

What is a routing metric?

The way a router determines the best path for a packet to take. Examples: hop count, bandwidth, load, delay, reliability

What is the difference between connection-oriented and connectionless systems?

A connection-oriented system has error checking built into it. It also establishes a connection with the receiving host before transmitting any data. A connectionless system does neither. It is a best-effort delivery system.

What is another name for a connectionless system?

packet switched

What is another name for a connection-oriented system?

circuit switched

What does IP rely on to determine whether packets have been lost and to request retransmission?

A Layer 4 connection-oriented protocol (like TCP)

Does a device have information on devices on other segments in its ARP table? Why or why not?

Routers do, but hosts don’t. Hosts can only “see” devices on the same network/subnet that the host is on. Routers, however, exchange tables of information with other routers so they can “see” devices on other networks. Hosts can get information on devices on other networks by going through a proxy ARP (e.g., router).

How does a device find the MAC address of another device on a different subnet?

It has to go through a proxy ARP device.

As a packet moves through the network, going from device to device and router to router, what piece of addressing information never changes?

The destination host’s IP address

What happens if a packet comes into a router that’s not directly connected to the destination network?

The router forwards the packet to the next router (hop) that is most likely to be connected to the destination network or has information on it.

What is the difference between dynamic and static routes?

Dynamic routes are those that routers learn automatically by exchanging routing tables. Routers are set up to exchange this information on a regular basis (e.g., every 30 seconds, 90 seconds, whatever). Static routes are entered into the router’s configuration file by the network administrator. Therefore, the only way a static route can be updated is if the administrator manually changes it in the router’s configuration file.

What is a stub network? Would it be routed statically or dynamically?

It’s a route to a network that only has one path to get there. It would normally be configured statically by the administrator.

What is one advantage of dynamically maintained networks VS statically maintained ones?

One advantage is that routers automatically update themselves. It takes little administrative overhead.

What is one disadvantage?

One disadvantage is security. Static routes are only known to the administrator(s) of the network. It’s difficult for outsiders to get this information.