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-Sa
Cisco 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
IPv6
32 bits
128 bits
Separated by a period
Separated by colons
133.15.6.4
RFC 791
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
 Router CPU load
 Sharing of a global IP address
 Poor network performance
 Security
 Remote access may be denied
if trying to access a server with
a private IP
 Scalability
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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.
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