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Planning the Addressing Structure

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

Copyleft 2012 Vincenzo Bruno (www.vincenzobruno.it)Released under Crative Commons License 3.0 By-SaCisco name, logo and materials are Copyright Cisco Systems Inc.

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Objectives

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Implementation of IP Addressing in the LAN

IP addresses are made up of 32 total bits– divided into 4 octets with a decimal separating them

– AKA: dotted decimal notation

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IP Addressing IP addresses are HIERARCHIAL (like a family tree)

– there are parents (networks)

– there are children (hosts)

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IP Address Classes Classes A – C

– assigned to users

Reserved: Class D (multicasts), Class E (experimental use)

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Private IP Addressing

Private IP addresses assigned by IETF (Internet Engineering Task Force)

– reduces the number of public IP addresses assigned to organizations

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Implementation of IP Addressing in the LAN

● Old 2 level hierarchy: Network –> Host

● Classful and Classless

● Default Subnet Mask for A, B, C classes

SUBNETS

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Traditional classful subnetting

the same number of host bits is used to designate the subnet ID for all the subnetworks

fixed number of subnets and a fixed number of hosts per subnet

known as fixed-length subnetting

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IP Addressing Practice

Class C address with a default subnet mask.– Question 1 – What is the default subnet mask

• 255.255.255.0

– Question 2 – How many usable hosts are available?

• 254

IPv4– Question 1 – Class which provides the largest number of

network bits?

• Class C

– Question 2 – Class which provides the largest number of hosts per network?

• Class A

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IP Addressing Practice

Class B address– Question 1 – What is the default subnet mask

• 255.255.0.0

– Question 2 – The IP address is 155.14.0.0 with a default subnet mask. What is the broadcast address for this network?

• 155.14.255.255

IPv4– Question 1 – The IP address is 192.168.4.0 with a subnet

mask of 255.255.255.224. What is the maximum number of sub networks that could be created.

• 6 networks

– Question 2 – The IP address is 192.133.219.0/27. What is the first usable host address in the first usable range?

• 192.133.219.33

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IP Versions

IPv4 IPv632 bits

Separated by a period

133.15.6.4

RFC 791

128 bits

Separated by colons

RFC 2460

More address space

Better address mgmt

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Custom Subnet Mask Custom subnet masks take bits from the host ID portion

of the IP address and add them to the default subnet mask

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Custom Subnet Mask Example /26

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VLSM and Classless Inter-Domain Routing (CIDR)

The original classful subnetting design required that all subnets of a single classed network be the same size

Fixed-length subnet masks can waste a significant number of IP addresses

VLSM addressing allows an address space to be divided into networks of various sizes

Routing information includes the IP address of the network

CIDR ignores network classes based on the value of the high-order bits

CIDR identifies networks based solely on the number of bits in the network prefix

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Using Network Address Translation in a Network (NAT)

Translates addresses from one network to another

Allows a large group of private users to access the Internet by sharing a small group of public IPs

– comparable to a company with a few phone lines, but many extensions

Developed to help save registered IP addresses

Provides security to PCs, servers, devices, etc

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Using Network Address Translation in a Network (NAT)

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Advantages & Disadvantages of NAT

Advantages Disadvantages Reuse of IP addresses

Sharing of a global IP address

Security

Scalability

Router CPU load

Poor network performance

Remote access may be denied if trying to access a server with a private IP

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NAT Terminology

Inside local network– any network connected to a router interface that is part of the

privately addressed LAN

Outside global network– any network attached to a router that is external to the LAN

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NAT: Inside and Outside Networks

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More NAT Terminology

Inside local address– Private IP address of a host on the local network

– Must be translated to a public IP before it can travel outside

Outside global address– Actual public IP address of an external host

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NAT: Inside and Outside Addresses

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More NAT Terminology - 2

Inside global address– IP address of inside host as it APPEARS to the outside

network

Outside local address– Destination address of packet while on the local network

– Usually the same as outside global address

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NAT: Inside and Outside Addresses - 2

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Static NAT Provides access to a local host from the Internet

Allows hosts on public network to access selected hosts on a private network

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Static NAT Example

The host with the IP address of 192.168.32.10 will always translate to 213.18.123.110.

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Dynamic NAT

Maps an unregistered IP address to a registered IP address from a group of registered IP addresses.

The host with the IP address of 192.168.32.10 will translate to the first available address in the range of 213.18.123.10 to 213.18.123.150

Limited by number of Outside Global addresses

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PAT (NAT Overload)

A form of dynamic NAT that maps multiple private IP addresses to a single registered IP address by using different ports. This is known also as PAT (Port Address Translation), single address NAT or port-level multiplexed NAT.

Each computer on the private network is translated to the same IP address (213.18.123.100), but with a different port number assignment.

Outside users are unable to initiate communication to a host if using PAT

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PAT (Overloading) Uses unique source port numbers in the inside global

IP address

Uses first available port number

These port numbers must be above 1024

Let's see how it works ->

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PAT: send

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PAT: receive

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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.