Different IP #'s required

Question

If I'm reading the setup instructions for my AE correctly, the AE gets the IP # that was provided by my internet provider, and my MBP gets a different #? My DSL modem took a lightning strike along with the AE which had been working great for the last 6 mos and now after replacing both units I can't get on the internet with the AE connected...direct to the modem is fine though. AE displays green light all the time.

Answer 1

Think of the internet as a city in which the people are computers. If you want to send somebody a letter, you need to know their address. On the internet, IP addresses are used instead of street addresses. "123 Oak Street" identifies an exact location in a city, and "192.0.32.10" identifies an exact location on the internet.
Generally your ISP will give you one IP address. The exact address may change from time to time, but whenever it does change, your ISP will let your router (or computer, if it's directly connected to your modem) know. The router (or computer) will update its settings accordingly, so you won't have to worry about what your IP address happens to be. The protocol by which all of this takes place is known as Dynamic Host Configuration Protocol, or DHCP.
Every computer connected to the internet needs an IP address, and two computers should not have the same address. So if you have more than one computer on your home network, it would appear that one IP address would not be sufficient to allow them all to connect to the internet.
Fortunately, routers take care of this problem for us. A router can use DHCP to give each computer on your network a unique "private" IP address. Going back to the city analogy, a network is like an office building. A private address would be something like "Room 123": to anyone in your office building, it identifies an exact location, but it's meaningless outside the office building. Private IP addresses generally start with "192.168" or "10.0", but a few other prefixes are also used. Inside your network, they identify your computer; outside your network, they are meaningless.
If the private IP addresses are meaningless outside your network, how can you communicate with computers outside your network? The router takes care of this using a process known as Network Address Translation (or NAT). A computer outside your network sends information addressed to your router's IP address, and the router routes the information to its intended destination (which it figures out by looking at the port number, a specific piece of the sent information).
This is why your computer may have a different IP address when connected to the modem than it does when connected to your AirPort: the AirPort is just doing its job.
Of course, if you can't connect to the internet, the AirPort may not be doing its job. It's also possible that the Express is doing a job, but not the right job.
AirPorts can be configured to use DHCP and NAT, just DHCP, or neither of them. Given your network setup, your Express needs to be using DHCP and NAT. You might want to check that your Express is configured to do so:
Open AirPort Utility and select your Express in the sidebar. Press "Manual Setup" and authenticate if necessary. Select "Internet" in the toolbar. For "Connection Sharing," you should have "Share a public IP address" selected.
Message was edited by: adsfushi72