ROUTING AND SWITCHING
Routing First-Step: Address resolution
William R. Parkhurst
09.29.2004
Rating: -4.20- (out of 5)
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The following is the third installment of a multi-part series on the fundamentals of routing. Each tip is excerpted from Routing First-Step by William Parkhurst, published by Cisco Press. Check back frequently for the next installment, or go to the main series page for all the installments.
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About the book
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Routing First-Step explains the basics of Internet routing in language all of us can understand. This book takes you on a guided tour of routing, starting with systems you are familiar with: the postal system, the telephone system, and the interstate highway system. From there, you'll learn routing simply and easily. Whether you are looking to take your first step into a career in networking or are interested only in gaining knowledge of the technology, this book is for you!
Author William R. Parkhurst, Ph.D., CCIE, manages the CCIE Development group at Cisco Systems. The CCIE Development group is responsible for all new CCIE written qualification and laboratory exams. Prior to joining the CCIE team, Bill was a Consulting Systems Engineer supporting the Sprint Operation. He first became associated with Cisco Systems while a Professor of Electrical and Computer Engineering at Wichita State University. In conjunction with Cisco Systems, WSU established the first CCIE Preparation Laboratory.
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Address resolution
An IP address is routable. Routers can use the network portion of an IP address to
make a delivery, or routing decision, to the destination network. Ethernet
addresses are not routable (unless every router knows how to reach every Ethernet
address). Ultimately, electronic data must be delivered to a host using the host's
Ethernet address. To do this, you need a protocol to determine, or resolve, the
Ethernet address associated with a host's IP address. There is an analogy for
address resolution that you are familiar with. Assume that you want to call your
friend Steve and you do not know his telephone number, but you know where he
lives. What do you use to resolve Steve's telephone number from his address? A
telephone book. With a computer network, you need to do essentially the same
thing when resolving between Ethernet and IP addresses. In Figure 3-5, there are
three LANs with four hosts each.
Figure 3-5 - Address Resolution Matches a Host's Ethernet Address
with a Host's IP Address
The Fullerton, Diversey, and Kostner LANs have been assigned networks
192.20.1.0, 192.20.2.0, and 192.20.3.0 (remember the 0 designates "this" network).
The host addresses on these LANs are .1, .2, .3, and .4. This is a shorthand
notation for IP addresses 192.20.1.1, 192.20.1.2, 192.20.1.3, and 192.20.1.4 on
the Fullerton LAN—and the same shorthand notation is used on the other two
LANs. Also notice that the three Ethernet interfaces on the access router have also
been assigned an IP address taken from the range of addresses associated with
each LAN.
This section uses the networks in Figures 3-5 and 3-6 to trace through the steps a
host uses to send data to a host on the same LAN and to a host on a different LAN.
Tables 3-6, 3-7, and 3-8 contain the IP and Ethernet addresses for the hosts and
router on the three LANs.
Table 3-6 - Fullerton LAN Address Associations
Table 3-7 Diversey LAN Address Associations
Table 3-8 Kostner LAN Address Associations
Intra-LAN Communication
In Figure 3-6, the host with IP address 192.20.1.1 on the Fullerton LAN wants to
send data to the host with IP address 192.20.1.2 on the same LAN. The source and
destination IP addresses are
Source: 192.20.1.1
Destination: 192.20.1.2
The source host knows that the destination IP address is on the same network
Because
Both source and destination network numbers are Class C.
Both Class C network numbers are the same; therefore, they both point to
the same network.
Figure 3-6 - Intra-LAN Address Resolution
The source host knows the destination IP address, but not the destination Ethernet
address. The source host needs to resolve the destination Ethernet address from
the destination IP address. This is accomplished by using the Address Resolution
Protocol (ARP). The source host sends an Ethernet broadcast to the switch. Like
the IP broadcast, an Ethernet broadcast is signified by setting the destination
Ethernet address to all 1s or FF:FF:FF:FF:FF:FF. The source Ethernet address is
set to the Ethernet address of the host sending the broadcast. The ARP message
contains the destination IP address or 192.20.1.2. When the Ethernet switch
receives the broadcast message, it is sent to all hosts on the network except for the host that sent the message. All hosts on the Fullerton LAN will receive the broadcast
and inspect the IP address in the message. If the IP address is not the IP
address of the host that received the message, the message will be ignored. When
the host with IP address 192.20.1.2 receives the ARP message, it will respond
back to the sender with its Ethernet address. Now the host at 192.20.1.1 has
resolved the Ethernet address for the host with IP address 192.20.1.2.
Host .1 on the Fullerton LAN receives the ARP request and stores that association
between the Ethernet and IP addresses for host .2 in an ARP table. Storing this
information allows host .1 to send additional messages to host .2 without having
to send an ARP request each time. An example of a typical ARP table is shown in
the following output:
Interface: 192.20.1.1
Internet Address -- Physical Address -- Type
192.20.1.2 -- 00-03-47-92-9C-70 -- dynamic
The physical address is the Ethernet address associated with IP address
192.20.1.2. Dynamic means that this association was learned using ARP.
At this point, you might be wondering why we have two addresses. Why not use
either the IP address or the Ethernet address. Why use both? The clue is in the
ARP table shown earlier. An Ethernet address is a physical address. It is "burned
in" to the Ethernet card and is sometimes referred to as a burned-in address (BIA).
An IP address is a logical address that was assigned to the host. In this case, the
host happens to use Ethernet for sending messages on the LAN. Other technologies
exist that can be used by the computers to send messages, such as Token ring
or Asynchronous Transfer Mode (ATM). If you use ATM on the Fullerton LAN
instead of Ethernet, you should expect that you are still able to send messages
between computers. An ATM address is 20 bytes while an Ethernet address is
6 bytes. In other words, the logical addressing (IP) should be independent of the
physical addressing (Ethernet, Token Ring, ATM). Does this sound familiar? In
Chapter 1, "Routing and Switching in Everyday Life," you learned a layered
model for the postal delivery system. (See Figure 3-7.)
Figure 3-7 - Layered Postal Delivery Model
For this model, you learned that the address should not be dependent on the contents,
and that the physical delivery should not be dependent on the address. The
layers in this model are independent. In the same way, you need a layered model
for the Internet. With what you've learned, you can start constructing the layer
model for the Internet. In Figure 3-8, the lowest layer is the network interface
layer.
Figure 3-8 - Partial Layered Internet Model
The network interface layer is concerned with the physical, electrical, and
addressing requirements for the particular technology used to deliver the messages.
The IP layer is a logical layer concerned with being able to route a message
between endpoints. The IP layer in the Internet model should be independent from
the network interface layer. This independence allows you to change the technology
used at the network interface layer without having to modify the IP layer.
Inter-LAN Communication
The host with IP address 192.20.1.1 on the Fullerton LAN wants to send data to
the host with IP address 192.20.3.3 on the Kostner LAN. The source and destination
IP addresses are
Source: 192.20.1.1
Destination: 192.20.3.3
The source host knows that the destination IP address is on a different network
Because
Both source and destination network numbers are Class C.
The source and destination Class C network numbers are not the same;
therefore, the source and destination computers are on different networks.
The host on the Fullerton LAN doesn't have to know how to get a message to the
host on the Kostner LAN. That is the function of the router. Because the source
host knows that the destination is on a different LAN or network, the host knows
that it must send the message to the router. Each host has been configured with the
IP address of the router interface that connects to their LAN. The router is the
gateway to the rest of the world, so the IP address of the router is called the default
gateway. In other words, if a host is sending a message to a different LAN, the
message must first be sent to the default gateway, or router, or last resort. The process
for inter-LAN communication is
- Send an ARP broadcast asking for the Ethernet address associated with the
default gateway (192.20.1.5).
- The router responds with the Ethernet address of the interface that is connected
to the source LAN (00-03-47-92-9C-73).
- Host 192.20.1.1 stores the router's IP address, and associated Ethernet
address in its local ARP table. The ARP table now contains
Interface: 192.20.1.1
Internet Address -- Physical Address -- Type
192.20.1.2 -- 00-03-47-92-9C-70 -- dynamic
192.20.1.5 -- 00-03-47-92-9C-73 -- dynamic
- The source host sends the message to the router.
- The router removes the source and destination Ethernet addresses from the
message and inspects the destination IP address (192.20.3.3).
- The router determines that the destination LAN is network 192.20.3.0 and
the destination host IP address is 192.20.3.3.
- The router sends an ARP request on the Kostner LAN asking for the Ethernet
address associated with IP address 192.20.3.3.
- Host 192.20.3.3 on the Kostner LAN sends an ARP reply containing its
Ethernet address to the router (00-03-49-C5-12-33).
- The router sends the message to the Ethernet address of host 192.20.3.3.
This process is similar to how mail is delivered. Figure 3-9 shows the flow of a letter
down the protocol stack that was developed for the postal system.
Figure 3-9 Flow of a Letter Down the Mail Protocol Stack
The letter is sent down to the Addressing Person, or Who layer where it is placed,
or encapsulated, in an envelope. The envelope is sent to the Addressing Where
layer and the state, city, street name, and street number information are added.
Remember that you have logically separated the Who from the Where information,
because the Who information is not used to deliver the letter. Finally, the envelope is passed to the Delivery layer where it is encapsulated or placed into whatever delivery means is being used (wagon, horse, truck, and so on).
As the letter makes its way through the postal delivery system, it passes through
one or more post offices. At each post office the letter is removed from the delivery
layer, and the destination address is inspected. Based on the destination
address, the post office makes a routing decision and the letter is again sent back
to the delivery layer and encapsulated (placed) in a new means of delivery.
Between the source of the letter and the letter's destination, the means of delivery
at each post office changes, but the source and destination addresses remain the
same. This process can be used to better understand the delivery of an electronic
message through a network. (See Figure 3-10.)
Figure 3-10 Flow of Data Down the IP Stack
Your application generates the data to be sent to another host. This data could be
an e-mail, an instant message, a request for a web page, and so on. The data is sent
to the first addressing layer where an application identifier is placed on the data.
Think of this as the Who part of the address. As with the postal system, this information
is not used to deliver the data, but to identify which application should
receive the data after it arrives at the destination. After the application identifier is
placed on the data, the next layer in the protocol stack adds the source and destination
IP addresses. Finally, the network interface layer adds the source and destination Ethernet addresses on the package (assuming the host is using Ethernet),and the package is transmitted toward the destination. In the Internet, the package of data is called a packet.
For intra-LAN communication, the receiving host inspects the destination Ethernet
address, and accepts the package if the host sees its own Ethernet address. If it
does, the Ethernet addresses (source and destination) will be stripped off, and the
remaining package will be sent to the IP layer. The IP layer inspects the destination
IP address to verify that the package is meant for this host. If it is, the IP
address is stripped off and sent to the application identification layer. After the
application has been identified, this information is stripped off and the data is sent
to the proper application.
For inter-LAN communication, the package is sent to the router. The router
inspects the destination Ethernet address and accepts the package if the router
sees its own Ethernet address. If it does, the Ethernet addresses (source and destination)
are stripped off and the remaining package is sent to the router's IP layer.
The destination IP address is inspected, and the router consults the routing table to
determine the interface it needs to use to send the package to the destination. The
router looks for the destination IP and Ethernet address association in the ARP
table. If the association is not in the ARP table, the router uses ARP to learn the
destination Ethernet address associated with the destination IP address. The package
is sent back to the network interface layer, and the package is encapsulated
using new source and destination Ethernet addresses. Finally, the package is sent
to the host, and the host will perform the same functions as mentioned for intra-
LAN delivery.
The package might have to travel through more than one router. At each router,
the same process takes place. The old Ethernet source and destination addresses
are removed, the IP routing table is consulted, and new source and destination
Ethernet addresses are applied. But no matter how many routers the package goes
through, the source and destination IP addresses do not change. Only the network
interface layer addresses change. The analogy between the layers of the mail and
data delivery systems is shown in Figure 3-11.
Figure 3-11 Mail and Electronic Data Delivery Protocol Stacks
All parts reproduced from the book Routing First-Step, ISBN 1587201224, Copyright 2005, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses. Visit www.ciscopress.com for a detailed description and to learn how to purchase this title.
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