Why Public IP? Why no Private ?

There may be some concerns where Network Address Translation (NAT) is not an option. Such concerns might be equipment or personnel limitations. Suffice it to say, If you are a customer of an ISP and want a larger network NAT is the way to go. An ISPthat claims that you need to have more Public addresses and charges extra for them, there is your answer!

My ISP charges a basic rate for service, and an additional fee for additional addresses. If they ever figure out that they have no mechanismto prevent additional PCs set up with DHCP, I will have to put one of my routers on and NAT!

Hi,

Straight NAT means that you must dish out addresses in the 192.168.0 network and you can only use 254 of them.

I would hope that most ISPs would want more than 254 clients...

Furthermore there will be issues of speed and reliability as this adds another level of complexity and possibilities for failure.

And finally there may be some issue for those that want to use NAT themselves. I'm not sure whether there is a problem using NAT to map from the 192.168.0 range back to the 192.168.0 range...but there could well be.

Also, I believe that under some circumstances, NAT simply doesn't work. Thankfully these are rare, but in the case where IP information is transferred within the data packet itself, there is no guarantee that NAT will correctly alter it in every case.

Hope this helps,
Cheers,
Tim.

I think ease of maintanence also. The ISP would have to configure all routers and hardware inbetween the customer and itself to let through privat IP's. With public IP's they may not have to touch their routers.

Security, I don't think ISP's want their customers to be on their private network unless absolutely necessary. Creating ACL's for all the routers who's customers are given private IP's is actually a lot more costly, time consuming, and much more difficult to manage than giving out public IP's.

I think the most likely answer is the issue of having to manage routing and name resolution issues, as well as support for clients using NAT. They would need additional servers, because they would have to provide DHCP/DNS services for the private addresses as well as the public ones. There might be bandwidth issues, too. However, there is not, as one answer above said, any limitation to the 192.168.x.x address range - you can also use the private range of 10.x.x.x, which allows enough addresses I would think for any ISP.

I guess I would agree with most of the answers. As an ISP I would not want the headache of managing my customers' IP addressing and NATing. #1 concern in my book. There is also a limit of addressing. BTW, 192.168.0.0 equals 256x256 -2 addresses. Notto mention the private 10.0.0.0 networks and 172.16-31.255.255 networks.

Let me bring to light some more issues that pose a problem.

Your services that you could use as a client would be much more limited. If you just wanted to have internet access for page viewing, AIM, and the like, Private addresses and NAT would work.

Everyone on a particular T-1/t-3 or what ever, would have the same IP address visable to the outside world. If you wanted to run and FTP or web server, you couldn't unless the ISP did static NAT and PAT. Imagine FTPing to 206.4.4.1 thathas 16,000 ip addresses NATed behind it. You would have to assign each person a specific port for their FTP server or web server. Then you run the risk of stepping on ports that could block services for other customers.

It is a whole lot simpler to DHCP a pool of IPs and route them than trying to NAT, PAT everything to keep all your customers happy. Some internet services just do not function in a NAT enviroment.

It COULD be done, but it would be a technical support nightmare. Your idea has merit, but is not very practical unfortunately. Blocks of IPs are not expensive. If you are paying for a T1, you can usually get a couple of class C's included. THey aren't portable, which means if you drop the service, you loose the ips. Or if you switch providers you have to re-number your networks.

ciscoboy has some good points. i also would look up the number aof hosts/service that NAT can operate for. There is a limit. Also some apps break if they cannot get a valid DNS lookup, which doesnot work for private space. Another issue, at least for US ISPs, are the occasional visits we get from the Secret Service. They show up and you have to positively identify who sent any messages, or traffic from your network. You must keep VERY good logs to get away with NAT. MUCH simpler to assign a single public address to each customer and then allow them to do NAT. It then becomes their headache to identify the user.

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