Network Layer (Part IV)

Overview

•

_{A router is a type of internetworking}

device that passes data packets between

networks based on Layer 3 addresses.

•

_{A router has the ability to make}

intelligent decisions regarding the best

path for delivery of data on the network.

•

_{In this chapter, you will learn how routers}

Overview

•

In addition, you will learn how devices on local-area

networks (LANs) use Address Resolution Protocol

(ARP) before forwarding data to a destination.

•

You will learn what happens when a device on one

network does not know the MAC address of a device

on another network.

•

You will learn that Reverse Address Resolution

Protocol (RARP) is the protocol a device uses when

it does not know its own IP address.

•

Lastly, you will learn the diference between routing

and routed protocols and how routers track distance

between locations.

Layer 3 Devices

: Routers

•

In networking, there are two addressing schemes:

one uses the MAC address, a data link (Layer 2)

address; the other uses an address located at the

network layer (Layer 3) of the OSI model.

•

An example of a Layer 3 address is an IP address.

A router is a type of internetworking device that

passes data packets between networks, based on

Layer 3 addresses.

Layer 3 Devices

: Layer 3

addresses

•

_{Bridges and switches use physical, or MAC}

addresses, to make data forwarding decisions.

Layer 3 Devices

: Layer 3

addresses

•

Physical, or MAC addresses, are usually assigned by

the NIC manufacturer and are hard-coded into the

NIC.

•

The network administrator usually assigns IP

addresses.

•

In fact, it is not unusual for a network administrator

to group devices together in the IP addressing

scheme, according to their geographical location,

department, or foor within a building.

•

Because they are implemented in software, IP

addresses are fairly easy to change.

•

Finally, bridges and switches are primarily used to

connect segments of a network.

Layer 3 Devices

: Unique

network numbers

•

_{Routers connect two or more}

networks, each of which must have a

unique network number in order for

routing to be successful.

Layer 3 Devices

: Unique

network numbers

•

_Example:

Another network, with a unique

network number - B - has four

devices attached to it. This network

is also attached to the same router,

but at a diferent interface. The IP

addresses of the devices on this

second network are B1, B2, B3, and

B4. The IP address of the router's

Layer 3 Devices

: Unique

network numbers

• _Example:

You want to send data from one network to another. The source network is A; the destination network is B; and a router is connected to networks A, B, C, and D. When data (frames), coming from network A, reaches the router, the router performs the following functions:

– It strips of the data link header, carried by the frame. (The

data link header contains the MAC addresses of the source and destination.)

– It examines the network layer address to determine the destination network.

– It consults its routing tables to determine which of its interfaces it will use to send the data, in order for it to reach its

Layer 3 Devices

: Unique

network numbers

Layer 3 Devices :

Router

interface/port

•

_{A router’s attachment to a network is}

called an interface; it may also be

referred to as a port.

Network-to-Network Communications :

Methods for assigning an IP address

•

_{After you have determined the addressing}

scheme for a network, you must choose

the method for assigning addresses to

hosts.

•

_{There are essentially two methods for}

assigning IP addresses -

static addressing

and

dynamic addressing

.

Network-to-Network Communications :

Methods for assigning an IP address

Static Addressing

If you assign IP addresses statically, you must go to each individual device and confgure it with an IP address.

• This method requires you to keep very meticulous records, because problems can occur on the network if you use

duplicate IP addresses.

• Some operating systems, such as Windows 95 and

Windows NT, send an ARP request to check for a duplicate IP address when they attempt to initialize TCP/IP.

• If they discover a duplicate, the operating systems will not initialize TCP/IP and will generate an error message.

Network-to-Network Communications :

Methods for assigning an IP address

•

_Dynamic

_Addressing

There are a few diferent methods that you can use to

assign IP addresses dynamically. Examples of these

are:

•

_{Reverse Address Resolution Protocol (RARP)}

Network-to-Network Communications :

Methods for assigning an IP address

• _{Let's look at an example where a source device wants to}

send data to another device.

• _{In our example the source knows its own MAC address,}

but is unable to locate its own IP address in its ARP table.

• _{In order for the destination device to retrieve the data,}

pass it to higher layers of the OSI model, and respond to the originating device, the source must include both its MAC address and IP address.

• _{Therefore, the source initiates a process called a RARP}

Network-to-Network Communications :

Methods for assigning an IP address

•

RARP uses the same packet format as ARP. But in a

RARP request, the MAC headers, IP headers, and

"operation code" are diferent from an ARP request.

•

The RARP packet format contains places for MAC

addresses of both destination and source.

•

The source IP address feld is empty. The broadcast

goes to all devices on the network; therefore the

destination IP address will be set to all binary 1s.

•

_{Workstations running RARP have codes in ROM that}

Network-to-Network Communications :

Methods for assigning an IP address

•

_{BOOTstrap Protocol (BOOTP)}

A device uses

BOOTstrap protocol

(BOOTP)

when

it starts up, to obtain an IP address.

•

_{BOOTP uses UDP to carry messages; the UDP}

message is encapsulated in an IP datagram.

•

_{A computer uses BOOTP to send a broadcast IP}

datagram (using a destination IP address of all

1s - 255.255.255.255).

•

_{A BOOTP server receives the broadcast and then}

Network-to-Network Communications :

Methods for assigning an IP address

• _{If it fnds its own MAC address in the destination address}

feld, then it takes the IP address in that datagram.

• _{Like RARP, BOOTP operates in a client-server}

environment, and only requires a single packet exchange.

• _{However, unlike RARP, which only sends back a 4 octet IP}

address, BOOTP datagrams can include the IP address, the address of a router (default gateway), the address of a server, and a vendor-specifc feld.

• _{One of the problems with BOOTP is that it was not}

designed to provide dynamic address assignment.

• _{With BOOTP you create a confguration fle that specifes}

Network-to-Network Communications :

Methods for assigning an IP address

• _{Dynamic Host Confguration Protocol (DHCP)}

Dynamic host confguration protocol (DHCP) has been proposed as a successor to BOOTP.

• _{Unlike BOOTP, DHCP allows a host to obtain an IP address}

quickly and dynamically.

• _{All that is required using DHCP is a defned range of IP}

addresses on a DHCP server. As hosts come online they contact the DHCP server and request an address.

• _{The DHCP server chooses an address and allocates it to}

that host.

• _{With DHCP, the entire computer’s confguration can be}

Network-to-Network Communications :

DHCP initialization sequence

•

_{When a DHCP client boots, it enters an}

_initialize

state

. It sends

DHCPDISCOVER

broadcast

messages, which are UDP packets with the port

number set to the BOOTP port.

•

_{After sending the DHCPDISCOVER packets, the}

client moves into the

select state

and collects

DHCPOFFER

responses from DHCP server. 

•

_{The client then selects the frst response it}

receives and negotiates lease time (the length of

time it can keep the address without renewing it)

with

the

DHCP

server

by

sending

a

DHCPREQUEST packet.

•

_{The DHCP server acknowledges a client request}

with

a DHCPACK

packet.

•

_{The client can now enter the}

_{bound state}

_and

Network-to-Network

Communications :

IP key components

•

_{In order for devices to communicate, the sending}

devices need both, the IP addresses and the

MAC addresses, of the destination devices.

•

_{When they try to communicate with devices}

whose IP addresses they know, they must

determine the MAC addresses.

•

_{The TCP/IP suite has a protocol, called ARP, that}

can automatically obtain the MAC address.

•

_{ARP enables a computer to fnd the MAC address}

Network-to-Network Communications :

•

_{A packet is divided into two major components:}

the header, which includes source and

destination addresses; and the data.

•

_{Other types of protocols have their own}

Network-to-Network Communications :

IP key components

•

_Note:

_{Another major component of IP is}

_Internet

Control Message Protocol (ICMP)

.

•

_{This protocol is used by a device to report a}

problem to the sender of a message.

•

_{For example, if a router receives a packet that it}

cannot deliver, it sends a message back to the

sender of the packet.

Network-to-Network Communications : Function

of the address resolution protocol (ARP)

•

_{Layer 3 protocols determine whether data passes}

beyond the network layer to higher levels of the OSI

model.

•

_{A data packet must contain both, a destination MAC}

address and a destination IP address.

•

_{If it lacks one or the other, the data will not pass from}

Layer 3 to the upper layers.

•

_{In this way, MAC addresses and IP addresses act as}

checks and balances for each other.

•

_{After devices determine the IP addresses of the}

Network-to-Network Communications : Function

of the address resolution protocol (ARP)

•

There are a variety of ways that devices can

determine the MAC addresses they need to

add to the encapsulated data.

•

_{Some keep tables that contain all the MAC}

addresses and IP addresses of other devices

that are connected to the same LAN.

•

_{They are called}

_{Address Resolution Protocol}

(

ARP)

tables

, and they map IP addresses to

Network-to-Network Communications : Function

of the address resolution protocol (ARP)

•

ARP tables are sections of RAM memory, in

which the cached memory is maintained

automatically on each of the devices.

Network-to-Network Communications : Function

of the address resolution protocol (ARP)

•

_{When a source determines the IP address for a}

destination, the source consults its ARP table in

order to locate the MAC address for the

destination.

•

_{If the source locates an entry in its table}