planning the addressing structure

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

Planning the Addressing Structure

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

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

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.


IP Addressing in the LAN IP addressing identifies hosts and network devices

IP address format: dotted-decimal notation

Hierarchical structure: network and host octets


IP Addressing in the LAN Address classes A, B and C: used to identify hosts or

networks

Address classes D and E: multicast and experimental uses


IP Addressing in the LANFirst 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


IP Addressing in the LAN Reserved address space for private networks

Private IPs are not routable on the Internet

Consumer networking devices give out private IPs through DHCP


IP Addressing in the LAN 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


IP Addressing in the LANClassful subnetting:

Use bits from the host space to designate a subnet ID

All resulting subnets use the same subnet ID


IP Addressing in the LANClassless subnetting features:

CIDR: identify networks based on the number of bits in the network prefix

VLSM: divide address space into networks of various sizes


IP Addressing in the LANCommunicating 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


Inside local network

Outside global network

NAT and PAT


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 PATIP 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 PATIPv6 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.

planning the addressing structure

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