CS 540
Computer Networks II
Sandy Wang
chwang_98@yahoo.com
11. NETWORK ADDRESS
TRANSLATION – NAT
Topics
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Overview
LAN Switching
IPv4
IPv6
Routing Protocols -- RIP, RIPng, OSPF
Routing Protocols -- ISIS, BGP
MPLS
Midterm Exam
Transport Layer -- TCP/UDP
Congestion Control & Quality of Service (QoS)
Access Control List (ACL)
Application Layer Protocols
Application Layer Protocols continue
Others – Multicast, SDN
Final Exam
Reference Books
• Cisco CCNA Routing and Switching ICND2 200-101 Official Cert
Guide, Academic Edition by Wendel Odom -- July 10, 2013.
ISBN-13: 978-1587144882
• The TCP/IP Guide: A Comprehensive, Illustrated Internet Protocols
Reference by Charles M. Kozierok – October 1, 2005.
ISBN-13: 978-1593270476
• Data and Computer Communications (10th Edition) (William
Stallings Books on Computer and Data Communications) by Williams
Stallings – September 23, 2013.
ISBN-13: 978-0133506488
http://class.svuca.edu/~sandy/class/CS540/
Agenda
• Basic Concept of
• Network Address Translation (NAT)
• Port Address Translation (PAT)
• Definition, Benefits, Availability
and Application Support
• NAT Concepts and Terminology
• Port Address Translation (PAT)
• NAT Technical Information
Private Network
• Private IP network is an IP network that is not directly connected to
the Internet
• IP addresses in a private network can be assigned arbitrarily.
• Not registered and not guaranteed to be globally unique
• Generally, private networks use addresses from the following
experimental address ranges (non-routable addresses):
• 10.0.0.0 – 10.255.255.255
• 172.16.0.0 – 172.31.255.255
• 192.168.0.0 – 192.168.255.255
6
Private Addresses
H1
10.0.1.2
H3
H2
H4
10.0.1.2
10.0.1.3
10.0.1.1
10.0.1.3
10.0.1.1
Private network 1
Private network 1
Internet
R1
128.195.4.119
128.143.71.21
R2
213.168.112.3
H5
7
Network Address Translation (NAT)
• RFC 1631
• A short term solution to the problem of the depletion of IP
addresses
• Long term solution is IPv6
• CIDR (Classless InterDomain Routing ) is a possible short term
solution
• NAT is another
• NAT is a way to conserve IP addresses
• Can be used to hide a number of hosts behind a single IP address
• Uses private addresses:
• 10.0.0.0-10.255.255.255,
• 172.16.0.0-172.32.255.255 or
• 192.168.0.0-192.168.255.255
8
Network Address Translation (NAT)
• NAT is a router function where IP addresses (and possibly port
numbers) of IP datagrams are replaced at the boundary of a private
network
• NAT is a method that enables hosts on private networks to
communicate with hosts on the Internet
• NAT is run on routers that connect private networks to the public
Internet, to replace the IP address-port pair of an IP packet with
another IP address-port pair.
9
Basic Operation of NAT
• NAT device has address translation table
• One to one address translation
10
Pooling of IP Addresses
• Scenario: Corporate network has many hosts but only a small
number of public IP addresses
• NAT solution:
• Corporate network is managed with a private address space
• NAT device, located at the boundary between the corporate network and the
public Internet, manages a pool of public IP addresses
• When a host from the corporate network sends an IP datagram to a host in
the public Internet, the NAT device picks a public IP address from the address
pool, and binds this address to the private address of the host
11
Pooling of IP Addresses
Private
network
Internet
Source
= 10.0.1.2
Destination = 213.168.112.3
Source
= 128.143.71.21
Destination = 213.168.112.3
NAT
device
private address: 10.0.1.2
public address:
H1
public address:
213.168.112.3
H5
Private
Address
Public
Address
10.0.1.2
Pool of addresses: 128.143.71.0-128.143.71.30
12
Supporting Migration between Network Service Providers
• Scenario: In CIDR, the IP addresses in a corporate network are obtained
from the service provider. Changing the service provider requires
changing all IP addresses in the network.
• NAT solution:
• Assign private addresses to the hosts of the corporate network
• NAT device has static address translation entries which bind the
private address of a host to the public address.
• Migration to a new network service provider merely requires an
update of the NAT device. The migration is not noticeable to the
hosts on the network.
Note:
• The difference to the use of NAT with IP address pooling is that the
mapping of public and private IP addresses is static.
13
Benefits
Corporate
Acquisition
Src Addr
10.6.1.20
Dest Addr
Acquisition
Before NAT
Outbound Packet
Src Addr
192.168.7.10
NAT
Dest Addr
Acquisition
After NAT
Outbound Packet
X
10.6.1.20
Dest Addr
Corporate
Src Addr
172.21.58.10
After NAT
Outbound Packet
10.6.1.51
Dest Addr
Corporate
Src Addr
10.6.1.51
Before NAT
Outbound Packet
• Problem: Merging corporations with conflicting private IP address space
need connectivity to each other and/or the Internet
• Solution: NAT provides transparent, scalable, and bi-directional connectivity
between corporate headquarters and acquisitions
Supporting Migration between network service Providers
15
IP Masquerading
• Also called: Network address and port translation (NAPT), port
address translation (PAT).
• Scenario: Single public IP address is mapped to multiple hosts in a
private network.
• NAT solution:
• Assign private addresses to the hosts of the corporate network
• NAT device modifies the port numbers for outgoing traffic
16
IP Masquerading
17
Load Balancing of Servers
• Scenario: Balance the load on a set of identical servers, which are
accessible from a single IP address
• NAT solution:
• Here, the servers are assigned private addresses
• NAT device acts as a proxy for requests to the server from the public network
• The NAT device changes the destination IP address of arriving packets to one
of the private addresses for a server
• A sensible strategy for balancing the load of the servers is to assign the
addresses of the servers in a round-robin fashion.
18
Load Balancing of Servers
19
Concerns about NAT
• Performance:
• Modifying the IP header by changing the IP address requires that NAT boxes
recalculate the IP header checksum
• Modifying port number requires that NAT boxes recalculate TCP checksum
• Fragmentation
• Care must be taken that a datagram that is fragmented before it reaches the
NAT device, is not assigned a different IP address or different port numbers
for each of the fragments.
20
Concerns about NAT
• End-to-end connectivity:
• NAT destroys universal end-to-end reachability of hosts on the Internet.
• A host in the public Internet often cannot initiate communication to a host in
a private network.
• The problem is worse, when two hosts that are in a private network need to
communicate with each other.
21
Concerns about NAT
• IP address in application data:
• Applications that carry IP addresses in the payload of the application data
generally do not work across a private-public network boundary.
• Some NAT devices inspect the payload of widely used application layer
protocols and, if an IP address is detected in the application-layer header or
the application payload, translate the address according to the address
translation table.
22
Agenda
• Basic Concept of NAT and PAT
• Definition, Benefits, Availability
and Application Support
• NAT Concepts and Terminology
• PAT
• NAT Technical Information
NAT Concepts
Outside Network
Inside Network
Inside
Interface
Inside Host
ip nat inside
NAT
Outside
Interface
ip nat outside
Outside Host
• An interface on the router can be defined as
inside or outside
• Translations occur only from inside to outside interfaces or vice
versa—never between the same type of interface
NAT Concepts
• NAT translations are static or dynamic
• Static translation are entered directly into the
configuration and are always in the translation table
• ip nat inside source static 10.6.1.20 171.69.68.10
• Dynamic translations use access lists to identify IP
addresses that NAT should create translations for
• ip nat inside source list 1 pool nat-pool
• access-list 1 permit 10.0.0.0 0.255.255.255
Static vs. Dynamic Translations
• Static translations
• When you need to be able to initiate
a connection from both the inside and
outside interfaces (e.g. SMTP, Web)
• Or you want a specific host to be translated
to a specific IP address
• Dynamic translations
• When you want to initiate a connection
from only the inside or only the outside
NAT Concepts
My
Network
Src Addr
Inside Local
Dest Addr
Outside Host
Before NAT
Outbound Packet
Src Addr
Inside Global
NAT
Dest Addr
Outside Host
Internet
After NAT
Outbound Packet
Inside Host
Outside Host
Dest Addr
Inside Host
Src Addr
Outside Local
After NAT
Inbound Packet
Dest Addr
Inside Host
Src Addr
Outside Global
Before NAT
Inbound Packet
• An IP address is either local or global
• Local IP addresses are seen in the inside network
• Global IP addresses are seen in the Outside network
Inside Local/Inside Global Example
My
Network
Src Addr
10.6.1.20
Dest Addr
Outside Host
Before NAT
Outbound Packet
Src Addr
171.69.68.10
NAT
Dest Addr
Outside Host
After NAT
Outbound Packet
10.6.1.20
Pro
---
Internet
Outside Host
Inside Global
Inside Local
171.69.68.10
10.6.1.20
NAT Address Pool
171.69.68.10
- - 171.69.68.11
171.69.68.12
171.69.68.13
Outside Local
---
Outside Global
---
For Outbound Packets an Address
Is Dynamically Allocated from the
NAT Address Pool
Inside Local/Inside Global Example
My
Network
Src Addr
10.6.1.20
Dest Addr
Outside Host
After NAT
Return Packet
Src Addr
171.69.68.10
NAT
Dest Addr
Outside Host
Internet
Before NAT
Return Packet
10.6.1.20
Outside Host
Inside Global
Global
Pro Inside
Pro
--- 171.69.68.1
----0
NAT Address Pool
171.69.68.10
- - 171.69.68.11
171.69.68.12
171.69.68.13
Inside Local
Local
Inside
--10.6.1.20
Outside Local
Local
Outside
-----
Outside Global
Global
Outside
-----
The NAT Address Translation Entry
in the Translation Table Is Used to
Translate Return Packets
NAT Terminology
• Inside local
• Configured IP address assigned to a host on the inside network;
address may be globally unique, allocated out of the private
address space defined in RFC 1918, or may be officially
allocated to some other organization
• Inside global
• The IP address of an inside host as it appears to the outside
host and network, “Translated IP Address”; addresses can be
allocated from a globally unique address space, typically
provided by the ISP (if the enterprise is connected to the global
Internet)
NAT Concepts
My Network
Internet
NAT
Inside Host
Outside Host
Src Addr
Inside Local
Src Addr
Outside Local
Before NAT
Outbound Packet
Src Addr
Inside Global
Dest Addr
Outside Global
After NAT
Outbound Packet
• Local IP addresses are seen on the inside network while
global IP addresses are seen on the outside network
NAT Concepts
Router# show ip nat translations
--Pro
Pro------------
----Inside
Inside Global
Global
-------171.69.70.15
171.69.70.15
--Inside
Local
Inside --Local
-------192.168.1.80
192.168.1.80
----Outside
Outside Local
Local
----------
----Outside
Outside Global
Global
----------
------- 171.69.68.10:1202
--tcp
----10.6.15.2:1202
--------204.71.200.67:80
204.71.200.67:80
------- 171.69.68.10:1460
--tcp
----10.8.20.25:146
0
--------204.71.200.69:80
204.71.200.69:80
• A NAT translation is 1 to 1 or many to 1
1 to 1 translations (NAT) assign a different
IP address for each translation
Many to 1 (PAT) translations can assign the same
IP address for each translation
PAT
Router# show ip nat translations
Pro
Inside Global
Inside Local
tcp
171.69.68.5:1405
10.6.15.2:1405
Outside Local
Outside Global
204.71.200.69:80 204.71.200.69:80
• PAT (Port Address Translation) includes ports
in addition to IP addresses
• Many-to-one translation
• Maps multiple IP addresses to 1 or a few IP addresses
• Unique source port number identifies each session
• Conserves registered IP addresses
• Also called NAPT in IETF documents
NAT vs. PAT
• NAT
• When there is sufficient number of IP addresses for 1 to 1
translations
• PAT
• When there are an insufficient number of IP addresses
available to translate all of the inside addresses
NAT Order of Operation
Inside Interface
Src Addr
Inside Local
Outside Interface
Dst Addr
Outside Local
Src Addr
Inside Global
Before NAT
Outbound Packet
Dest Addr
Inside Local
Src Addr
Outside Local
After NAT
Inbound Packet
Dest Addr
Outside Global
After NAT
Outbound Packet
Routing
NAT
Dest Addr
Inside Global
Src Addr
Outside Global
Before NAT
Inbound Packet
• NAT always checks translation table for entry before
access lists
• For a full NAT order of operation see
http://www.cisco.com/warp/public/556/5.html
Summary
• NAT provides transparent and
bi-directional connectivity
between networks having arbitrary addressing schemes
• NAT eliminates costs associated
with host renumbering
• NAT eases IP address management
• NAT enhances network privacy