CCNA Discovery 2 - Chapter 4

Planning the
Addressing Structure

Working at a Small-to-Medium Business or ISP –
Chapter 4

Version 4.1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1

Objectives
 Describe how IP Addressing is implemented in the
LAN.
 Subnet a given network to allow for efficient use of IP
address space.
 Explain how Network Address Translation (NAT) and
Port Address Translation (PAT) are used in a network.

© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 2

IP Addressing in the LAN
 IP addressing identifies hosts and network devices
 IP address format: dotted-decimal notation
 Hierarchical structure: network and host octets


 Address classes A, B and C: used to identify hosts or
networks
 Address classes D and E: multicast and experimental
uses


First octet bit patterns and classes:
 Class A: first bit is always 0
 Class B: first two bits are always 1 and 0
 Class C: first three bits are always 1, 1 and 0


 Reserved address space for private networks
 Private IPs are not routable on the Internet
 Consumer networking devices give out private IPs
through DHCP


 RFC 917, Internet Subnets
 Subnet mask separates network bits from host bits
 Routers read subnet masks left to right, bit for bit
– Bits set to 1 are part of the network ID
– Bits set to 0 are part of the host ID


Classful subnetting:
 Use bits from the host space to designate a
subnet ID
 All resulting subnets use the same subnet ID


Classless subnetting features:
 CIDR: identify networks based on the number of bits in
the network prefix
 VLSM: divide address space into networks of various
sizes


Communicating between subnets:
 Each subnet is a separate network
 Router is needed to communicate between them
 Each router interface is the default gateway for its
subnet


NAT and PAT
 Network address translation (NAT) allows private users
to access the Internet by sharing one or more public IP
addresses


NAT and PAT
 NAT operation is transparent to users
 Benefits include improved security and scalability


NAT and PAT
 Inside local network
 Outside global network


NAT and PAT
 Dynamic NAT assigns outside global addresses from a
pre-defined pool
 Static NAT assigns a permanent registered global IP to
an individual private host IP


NAT and PAT
 PAT translates multiple local addresses to a single
global IP address


NAT and PAT
 PAT conversations use a unique temporary IP address
and port number combination
 Port numbers above 1024
 Maximizes use of addresses and security


NAT and PAT
IP Nat issues:
 Additional workload to support IP addresses and port
translations
 Careful network design and equipment selection
 Accurate configuration


NAT and PAT
 Temporary solutions to address depletion: subnetting,
private IP addressing, and NAT
 Improvements proposed by using IPv6:
– More address space and better space management
– Easier administration
– Support for advanced network capabilities


NAT and PAT
IPv6 address notation:
 128 bits
 32 hexadecimal digits
 Three-part hierarchy: global prefix, subnet and
interface ID


Summary
 IP addressing can be tailored to the needs of the
network design through the use of custom subnet
masks.
 Classless subnetting gives classful IP addressing
schemes more flexibility through the use of variable
length subnet masks.
 Network Address Translation (NAT) is a way to shield
private addresses from outside users.
 Port Address Translation (PAT) translates multiple local
addresses to a single global IP address, maximizing the
use of both private and public IP addresses.


CCNA Discovery 2 - Chapter 4

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