Ch21 network layer final

21.1
Chapter 21
Network Layer:
Address Mapping,
Error Reporting,
and Multicasting
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

21.2
21-1 ADDRESS MAPPING
The delivery of a packet to a host or a router requires
two levels of addressing: logical and physical. We need
to be able to map a logical address to its corresponding
physical address and vice versa. This can be done by
using either static or dynamic mapping.
Mapping Logical to Physical Address
Mapping Physical to Logical Address
Topics discussed in this section:

McGraw-Hill ©TheMcGraw-HillCompanies,Inc., 2000
Address Resolution
Protocol (ARP)

 ARP (Address Resolution Protocol) is a network protocol used
to find out the hardware (MAC) address of a device from an
IP address.
 It is used when a device wants to communicate with some
other device on a local network (for example on an Ethernet
network that requires physical addresses to be known before
sending packets).
 The sending device uses ARP to translate IP addresses to
MAC addresses.
 The device sends an ARP request message containing the IP
address of the receiving device.
 All devices on a local network segment see the message, but
only the device that has that IP address responds with the
ARP reply message containing its MAC address.
 The sending device now has enough information to send the
packet to the receiving device.
 ARP request packets are sent to the broadcast addresses
(FF:FF:FF:FF:FF:FF for the Ethernet broadcasts and
255.255.255.255 for the IP broadcast).

 Let's say that Host A wants to communicate with host B.
 Host A knows the IP address of host B, but it doesn't know
the host B's MAC address.
 In order to find out the MAC address of host B, host A sends
an ARP request, listing the host B's IP address as the
destination IP address and the MAC address of
FF:FF:FF:FF:FF:FF (Ethernet broadcast).
 Switch will forward the frame out all interfaces (except the
incoming interface).
 Each device on the segment will receive the packet, but
because the destination IP address is host B's IP address,
only host B will reply with the ARP reply packet, listing its
MAC address.
 Host A now has enough information to send the traffic to host
B.

21.7
Figure 21.1 ARP operation

Mapping Physical to Logical Address:
RARP, BOOTP, and DHCP
 There are occasions in which a host knows its physical address, but needs
to know its logical address. This may happen in two cases:
 1. A diskless station is just booted. The station can find its physical address
by checking its interface, but it does not know its IP address.
 2. An organization does not have enough IP addresses to assign to each
station; it needs to assign IP addresses on demand. The station can send
its physical address and ask for a short time lease.
21.8

 RARP
 Reverse Address Resolution Protocol (RARP) finds the logical address for a
machine that knows only its physical address.
 The machine can get its physical address (by reading its NIC, for example),
which is unique locally. It can then use the physical address to get the
logical address by using the RARP protocol. A RARP request is created and
broadcast on the local network. Another machine on the local network that
knows all the IP addresses will respond with a RARP reply. The requesting
machine must be running a RARP client program; the responding machine
must be running a RARP server program.
 BOOTP
 The Bootstrap Protocol (BOOTP) is a client/server protocol designed to
provide physical address to logical address mapping
21.9

 DHCP
 BOOTP is not a dynamic configuration protocol. When a client requests its
IP address, the BOOTP server consults a table that matches the physical
address of the client with its IP address. This implies that the binding
between the physical address and the IP address of the client already
exists. The binding is predetermined. However, what if a host moves from
one physical network to another? What if a host wants a temporary IP
address? BOOTP cannot handle these situations because the binding
between the physical and IP addresses is static and fixed in a table until
changed by the administrator. BOOTP is a static configuration protocol. The
Dynamic Host Configuration Protocol (DHCP) has been devised to provide
static and dynamic address allocation that can be manual or automatic.
 Dynamic Address Allocation DHCP has a second database with a pool of
available IP addresses. This second database makes DHCP dynamic. When
a DHCP client requests a temporary IP address, the DHCP server goes to
the pool of available (unused) IP addresses and assigns an IP address for a
negotiable period of time.
21.10

21.11
An ARP request is broadcast;
an ARP reply is unicast.
Note

21.12
DHCP provides static and dynamic
address allocation that can be
manual or automatic.
Note

21.13
21-2 ICMP
The IP protocol has no error-reporting or error-correcting
mechanism. What happens if something goes wrong? What
happens if a router must discard a datagram because it cannot
find a router to the final destination?
These are examples of situations where an error has occurred
and the IP protocol has no built-in mechanism to notify the
original host. The IP protocol has no error-reporting or error-
correcting mechanism. The IP protocol also lacks a mechanism
for host and management queries. The Internet Control
Message Protocol (ICMP) has been designed to compensate for
the above two deficiencies. It is a companion to the IP protocol.

21.14
ICMP always reports error messages to
the original source.
Note

21.15
21-3 IGMP
The IP protocol can be involved in two types of
communication: unicasting and multicasting. The
Internet Group Management Protocol (IGMP) is one
of the necessary, but not sufficient, protocols that is
involved in multicasting. IGMP is a companion to the
IP protocol.

Ch21 network layer final

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