Famous IP Address 192.168.1.X

Answered Question
Mar 2nd, 2007

hi mates :)

there are two IP addresses I see wherever I look:

192.168.1.x (Class C)

172.16.x.x (Class B)

Is there a specific reason makes those addresses special? (Like they are not used as public internet address)

tx in advance for helping

I have this problem too.
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Correct Answer by smothuku about 9 years 6 months ago

Hi Jon ,

Thanks for your info.I know that NAT and PAT are used for translating private ip adderess to public addresses to connect to internet.

By using private ip addresses alone we can't connect to internet.That's why NAT came into the picture.



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Overall Rating: 5 (2 ratings)
smothuku Fri, 03/02/2007 - 03:48

Hi ,

Private Address Space

The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of IP address space for private networks: - - -

The first block is a single class A network number, the second block is a set of 16 contiguous class B network numbers, and the third block is a set of 255 contiguous class C network numbers.

If you decide to use private address space, you don't need to coordinate with IANA or an Internet registry. Addresses within this private address space will only be unique within your network. Remember, if you need globally unique address space, you must obtain addresses from an Internet registry.

In order to use private address space, determine which hosts do not need to have network layer connectivity to the outside. These hosts are private hosts, and use private address space. Private hosts can communicate with all other hosts within the network, both public and private, but they cannot have IP connectivity to any external host. Private hosts can still have access to external services via application layer relays.

All other hosts are public and use globally unique address space assigned by an Internet registry. Public hosts can communicate with other hosts within the network, and can have IP connectivity to external public hosts. Public hosts do not have connectivity to private hosts of other networks.

Because private addresses have no global meaning, routing information about private networks is not propagated on outside links, and packets with private source or destination addresses should not be forwarded across such links. Routers in networks that do not use private address space, especially those of Internet service providers, should be configured to reject (filter out) routing information about private networks. This rejection should not be treated as a routing protocol error.

Indirect references to such addresses (like DNS Resource Records) should be contained within the network. Internet service providers should take measures to prevent such leakage.

Advantages and Disadvantages of Using Private Address Space

The obvious advantage of using private address space for the Internet at large is to conserve the globally unique address space. Using private address space also gives you greater flexibility in network design, since you will have more address space available than you could get from the globally unique pool.

The primary disadvantage of using private address space is that you have to renumber your IP addresses if you want to connect to the Internet.



Jon Marshall Fri, 03/02/2007 - 03:53

Hi Satish

Great post but i would take issue on one point. It is not a disadvantage that if you use a private address range if you want to connect to the internet you have to renumber. That's what NAT is for and the way that most enterprises connect to the internet.

I didn't want the original poster thinking to connect to the Internet all your internal addressing had to use public addressing.


Correct Answer
smothuku Fri, 03/02/2007 - 04:12

Hi Jon ,

Thanks for your info.I know that NAT and PAT are used for translating private ip adderess to public addresses to connect to internet.

By using private ip addresses alone we can't connect to internet.That's why NAT came into the picture.



Jon Marshall Fri, 03/02/2007 - 03:50


Yes it is because they are private and to that list i would add (Class A).

Many companies use these address ranges internally on their networks as there just aren't enough Public IP addresses to go round. A company could use public addressig on their network if they wanted to, even if that address range didn't belong to them but any mistake and these addresses could "leak".

A lot of examples you see use these addresses also simply because it is safer to use this addressing.



Danilo Dy Fri, 03/02/2007 - 04:52

Some corporate network IP Addressing in some part of the world start with either 168.a.b.c, 8.a.b.c, 80.a.b.c, or 88.a.b.c good luck with those :)

jwharrison Fri, 03/02/2007 - 20:14

If there were no private network address ranges, then all systems would be on the INTERNET...and very unprotected. Private ranges are used for business and small networks, and then NAT is used to all access to the internet.



and if you want to know the math behind it all.


Danilo Dy Fri, 03/02/2007 - 23:01

I know those things you said and more than that.

What I mean in the earlier post, is there are some who use those numbers for luck (Chinese Feng Shui) without thinking the consequences. I do encountered a few myself :)

ryancolson Sun, 06/17/2007 - 08:08

in theory you cna use public addresses internally, but you will likely not be able to communicate with those real addresses on the internet. Also, you would still need to use NAT/PAT to get out. However(im not 100% sure on this as i am still new to this), you MIGHT be able to use public addresses internally if you NAT both ways instead of just source nating, however I am not 100% sure how this works or how to set it up.

srue Mon, 06/18/2007 - 05:49

wow. no mention of rfc 1918 yet?

RFC 1918 addresses the issue of these three 'private' network addresses.


There's no reason you shouldn't be able to talk to another network engineer and refer generically to an RFC1918 address and have them not know what you mean.

There is also a multicast equivalent and IPv6 equivalent of private addresses, but let's not get ahead of ourselves.

Then there is RFC 2827 which addresses why it's a good idea to block RFC 1918 addresses from entering your network from the Internet (as well as other addresses).

dave.keith Tue, 06/19/2007 - 07:51

Of course you can use routeable IPs internally Ryan, it's been done since the beginning of IP. It is the basis of how Internet Hosts communicate, extremely fundamental. NAT/PAT came along much later in life.

I too was a bit amazed that the answer to the original question was not simply "rfc1918".


ryancolson Tue, 06/19/2007 - 07:56

What i meant was more along the lines f using NAT on both the source and destination side when you are communicating with a network that has an address space that overlaps yours

acomiskey Tue, 06/19/2007 - 08:02


I'm not sure that's what ryan meant exactly. I think what he was saying is you could use non rfc1918 addresses internally even if they weren't your ip's. That's why he said...

"but you will likely not be able to communicate with those real addresses on the internet"

ryancolson Tue, 06/19/2007 - 08:07

well that too but mainly what i meant is where you not only translate the source for outbound and destination for inbound, but where you also translate the destination for outbound and the source for inbound

schroednet Mon, 06/18/2007 - 09:23


They are used for private "inside" IP

addressing. Then if you require any outside access, one can NAT a public IP over to one of the private side IP addresses.

This way outsiders can only ping/see your public IP address and have no idea what device or service is on the other side unless you allow it to be seen. This also cuts down on the number of public IP addresses one has to acquire.

P.S. - Don' for the the class A


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