What is Network AddressTranslation?
2. Port multiplexing
The new Network Address Translation (NAT) functionality in the Cisco
routers allows privately addressed networks to connect to public networks such
as the Internet. When the privately addressed network (the "inside")
sends a packet through the NAT router, the addresses are converted to legal
(registered) IP addresses before being passed to the "outside". Many
sites have been using pass-through firewall gateways to provide this
functionality. Well, it's now available in your Cisco router!
I've been encountering more and more sites that need to connect to
others, where both parties are using private addressing. Network 10.0.0.0 is
particularly popular (along the "more is better" line of thinking?).
Two sites that have both used the same private network number have to either
coordinate addressing or re-address to avoid conflicts. Now, with NAT, the same
address ranges can be used multiple times.
We've already mentioned that NAT cures duplicate address ranges
without readdressing host computers. The translation done by NAT can be either
static or dynamic. Static translation is where we specify a lookup table, and
one inside address is turned into one pre-specified outside address. Dynamic is
where we tell the NAT router what inside addresses need to be translated, and
what pool of addresses may be used for the outside addresses. There can be
multiple pools of outside addresses. ICMP host unreachable messages are used
when addresses run out.
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| NAT with Peplink Balance Deployed in Drop-in Mode |
With NAT, multiple internal hosts can also share a single outside IP
address, which conserves address space. This is done by port multiplexing:
changing the source port on the outbound packet so that replies can be directed
back to the appropriate machine.
3. Load distribution
You can also do load distribution via NAT: have one
external address (perhaps your Web server's name, www.cisco1900router.com, maps
to this address). Then round-robin between different inside machines, so that
incoming new connections are distributed across several machines. (Since each
connection may involve state information, a given connection has to remain on
one machine.)
4. Readdressing
Organizations that change service providers are now typically not
allowed to take their address with them (because exceptions to CIDR addressing
blocks have become a problem). NAT solves this by allowing re-addressing to
occur at the gateway, allowing time to convert internal hosts to the new
network number.
Additionally, NAT also enhances security---internal network topology
and addresses are hidden from the outside world. The only thing NAT really
can't do much about is sloppily-written applications, with hard-coded raw IP
addresses.
Disadvantages of NAT
Address translation is not practical for large numbers of internal
hosts all talking at the same time to the outside world. NAT just won't work
well at a large scale.
Besides, performance may be a consideration. Currently, NAT only
runs on Cisco 7500 routers with the RSP. Even there, NAT causes process
switching on its configured interfaces. You can think of this as if the CPU has
to look at every packet, deciding whether or not to translate it, and whether
to alter the IP header, or possibly the TCP header. One doubts that this will
be easily cache-able.
In terms of this problem, I refer you to the Cisco Command
Configuration Guide, which is very lucid. The NAT section of the IP chapter is
particularly well-written and clear. It has good illustrations and explains in
detail what happens as a packet is forwarded through the NAT router.
Here's a minimal sample configuration for static address
translation. We assume Ethernet 0 is "inside" and Serial 0 is
"outside". Private network 10.0.0.0 is used inside, and 192.1.1.0
outside. We'll translate 10.1.2.3 to 192.1.1.2 (and vice versa). The words
"inside source" emphasize that the inside source address is what's
getting changed.
ip nat inside source static 10.1.2.3 192.1.1.2
interface ethernet 0
ip address 10.1.2.1 255.255.255.0
ip nat inside
interface serial 0
ip address 192.1.1.1 255.255.255.0
ip nat outside
You may add address mappings or inside or outside interfaces as necessary.
Let's look at dynamic (pooled) translation with the same network and addresses as before. We'll set up a pool of addresses, translating sources in the range 10.1.2.0 through 10.1.2.255 to the range 192.1.1.10 through 20. The access list indicates what source addresses can be translated. The idea of the third line is that inside source addresses matching list 20 get translated to addresses from the pool named LegalPool. It pretty much says that, doesn't it!
ip nat pool LegalPool 192.1.1.10 192.1.1.20
access-list 20 permit 10.1.2.0 0.0.0.255
ip nat inside source list 20 pool LegalPool
interface ethernet 0
ip address 10.1.2.1 255.255.255.0
ip nat inside
interface serial 0
ip address 192.1.1.1 255.255.255.0
ip nat outside
And then there's address overloading (port multiplexing). To obtain a sample configuration for doing port multiplexing, add the word "overload" at the end of the "ip nat inside source" line. This gives the router permission to start sharing addresses.
ip nat inside source list 20 pool LegalPool overload
You can configure outside source address translation similarly, changing "inside source" to "outside source" in the above examples.
Let's look at how to do static outside address translation, supposing subnet 10.1.5.0 occurs both inside and outside (we're connecting to another company here). We only need to talk to the outside machine 10.1.5.3, and we'll readdress it as private address192.168.1.1 on the inside (if we use 10.1.5.x, we have more complex routing issues to think about). This might call for something like the following.
ip nat outside source static 10.1.5.3 192.168.1.1
interface ethernet 0
ip address 10.1.2.1 255.255.255.0
ip nat inside
interface serial 0
ip address 10.1.3.1 255.255.255.0
ip nat outside
I'm ignoring routing issues here (as in, why does the inside net think network 192.168.1.0 is back through this NAT router?). This sort of thing can also be done dynamically, using a pool of inside addresses, and an access list to match the outside source address, much like our earlier dynamic example. You can even translate both ways, changing the source addresses as packets go through the router in either direction. Just combine the "inside source" and "outside source" configurations!
Finally, there's TCP Load Distribution. We define a pool of addresses containing the real hosts' addresses, ending with "type rotary". The access list now permits the IP address of the virtual host, i.e. what the outside world thinks is the host address. So the virtual host is 192.1.1.50, with the real hosts being 10.1.2.20 through 30.
ip nat pool ServerPool 10.1.2.20 10.1.2.30 type rotary
access-list 30 permit 192.1.1.50 0.0.0.0
ip nat inside destination list 30 pool ServerPool
interface ethernet 0
ip address 10.1.2.1 255.255.255.0
ip nat inside
interface serial 0
ip address 192.1.1.1 255.255.255.0
ip nat outside
There are also commands to tune address translation. The dynamic translations time out after non-use; the timeout period is configurable. (Without overloading, the timeout is 24 hours). The number is how many seconds before timeout.
IP NAT translation time out 3600
With overloading, there is finer control, for UDP, DNS, and TCP, also Finish and Reset packets:
ip nat translation udp-timeout 3600
ip nat translation dns-timeout 3600
ip nat translation tcp-timeout 3600
ip nat translation finrst-timeout 3600
(Please note I am not recommending 3600 seconds as a timeout, just using it to demonstrate the syntax of the command.)
Commands to Manage NAT
To clear the dynamic translation table before timeout occurs:
Clear ip nat translation *
You can also clear specific entries. To see what's there:
show ip nat translations
show ip nat translations verbose
There's also:
show ip nat statistics
There are also NAT debug commands. These include:
debug ip nat
debug ip nat 110
debug ip nat detailed
(Here the 110 is an access list number governing which NAT packets to display, as with other debug commands in recent IOS releases).
Don't use these debug commands in a production router, you'll have more output than you could possibly have wanted. They might be darn handy in a test scenario when you're trying to see how NAT works, or what it does!
This material is originally from:
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