1Andrew Smith

NAT and DHCP (Network Address Translation and Dynamic

Host Configuration Protocol )

2Andrew Smith

NAT and DHCP

This session will cover• The sharing of one (or a few IP address) by translation

• Reduction of demand for ‘internet’ IP addresses

• The automatic issue of IP addresses to a host on a large scale system, without the need to configure the host

3Andrew Smith

NAT and DHCP

WE will be working on a derivation of the standard WAN Mesh used in all Day Schools

4Andrew Smith

NAT and DHCP

NAT (Network Address Translation)• Used to aggregate (collect) a range of

addresses, usually private into one (or few) externally recognisable addresses

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NAT and DHCP

NAT (Network Address Translation)• Cisco defines the following NAT terms:

– Inside local address – The IP address assigned to a host on the inside network. The address is usually not an IP address assigned by the Internet Network Information Center (InterNIC) or service provider. This address is likely to be an RFC 1918 private address.

– Inside global address – A legitimate IP address assigned by the InterNIC or service provider that represents one or more inside local IP addresses to the outside world.

– Outside local address – The IP address of an outside host as it is known to the hosts on the inside network. – Outside global address – The IP address assigned to a host on the outside network. The owner of the host assigns this

address.

6Andrew Smith

NAT and DHCP

NAT (Network Address Translation)

Inside Local

Inside global address10.x.x.x translated as

103.0.2.9 Outside global

address

Outside global address, when

128.23.2.2 is seen as 10.254.254.254 to the local hosts

7Andrew Smith

NAT and DHCP

NAT (Network Address Translation)• Normally NAT will translate a range of address to a network• For example you could have 10.x.x.x translated to 101.1.1.1 – 101.1.1.5• PAT (Port Address Translation) uses one address and sends each

connection on a new (unique TCP port) with the potential of 65535 though most systems (routers) can only cope with 2000

8Andrew Smith

NAT and DHCP

NAT (Network Address Translation)• PAT (Port Address Translation) uses one address and sends each

connection on a new (unique TCP port) with the potential of 65535 though most systems (routers) can only cope with 2000

• This means 10.x.x.x, 172.16.x.x or 192.168.x.x can be translated through one address, most of your ADSL/Broadband routers do this

9Andrew Smith

NAT and DHCP

NAT (Network Address Translation)• Static translation, the creation of a 1:1 relationship between and external

and internal address, completed when you have an externally facing server (like mail or web) on a NAT translated network

• Dynamic translation, when a host is allocated an address or ‘port’ via NAT/PAT

10Andrew Smith

NAT and DHCP

NAT (Network Address Translation)• Static translation, the creation of a 1:1 relationship between and external

and internal address, completed when you have an externally facing server (like mail or web) on a NAT translated network

• Dynamic translation, when a host is allocated an address or ‘port’ via NAT/PAT

11Andrew Smith

NAT and DHCP

DHCP (Dynamic Host Configuration Protocol)• Allowing a ‘large’ or ‘small’ scale system to issue addresses to all hosts

(from workstations to servers and printers) rather than the admin having to set the IP address on each device.

• The address is leased, for a defined period of time.• Your ISP may do this, your ADSL/Broadband router may do this

12Andrew Smith

NAT and DHCPDHCP (Dynamic Host Configuration Protocol)• Done using UDP, on ports 67 (DHCP server) and 68 (DHCP host)• The host must receive

– An issued IP address

– The Default Gateway address (which may also be the router, but please be aware a DHCP server can run autonomously)

– Subnet mask

– DNS server address, which may be internal or externally available • WINS, Domain name and other parameters can also be issued

13Andrew Smith

NAT and DHCP

DHCP (Dynamic Host Configuration Protocol)• Addresses may be issued by

– Automatic allocation – DHCP assigns a permanent IP address to a client. – Manual allocation – The IP address for the client is assigned by the administrator.

DHCP conveys the address to the client.

– Dynamic allocation – DHCP assigns, or leases, an IP address to the client for a limited period of time.

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NAT and DHCP

NAT and DHCP configuration

Please ensure your router (s) are configured as per the normal infra-structure

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NAT and DHCP

NAT configurationaccess-list 1 permit 172.16.0.0 0.0.0.255

ip nat inside source list 1 interface FastEthernet0/0 overload

interface fastethernet 0/0

ip nat inside

interface serial0/0

ip nat outside

16Andrew Smith

NAT and DHCP

DHCP configuration

service dhcp

ip dhcp pool sanfranciscopool

network 172.16.0.0 255.255.255.0

default-router 172.16.0.1

dns-server 172.16.0.2

domain-name widgetsrus.co.uk

netbios-name-server 172.16.0.3

Exit

ip dhcp excluded-address 172.16.0.1 172.16.0.10

17Andrew Smith

NAT and DHCP

Testing the configurationTo check DHCPRun ‘ipconfig’ to check the DHCP address has been obtainedRun ‘ipconfig /release’ to remove an automatic addressing.Run ‘ipconfig /renew’ to receive a new DHCP licensed address

To check NATYou must ping the nearside interface on the afar router from the host on the LANFor example, from the computer on the Madrid LAN you must ping 100.0.0.1 –t, note the ‘-t’ means continuous (no timeout)

On each router, issue the ‘clear ip nat translation *’ command‘show ip nat translations’ command

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NAT and DHCP

19Andrew Smith

NAT and DHCP