Sharing a Mac Internet Connection Through Airport

Ulrik Sandberg — Sat, 20 Jan 2007 22:02:03 +0000

I just solved a networking problem on my Macs after hours of trying. For the benefit of others, I'll describe here the problem and the solution.

Setup

Cable modem from ISP, network cable to a firewall, the private end of the firewall via network cable on to a switch. Pretty basic, I guess. No wireless base stations or anything. Just an iMac connected via network cable to the switch.

Problem

Share the iMac's network cable connection through its Airport wireless capability. The client is a MacBook Pro. Basically, I just want to get Internet access on my laptop via the iMac. Sounds simple, but hey, it took me half a day and plenty of research to figure out.

Useful pieces of information gathered during this adventure

Turning on Internet Connection Sharing (ICS) will start a DHCP server.
Turning on ICS will create a full Airport network (not a computer-to-computer network).
Turning on ICS will create a new network configuration for 10.0.2.1, which I understand is the same address as the Airport base stations use.

Solution

On the iMac, set the Airport network to "Use DHCP with manual address" and set the IP to 10.0.2.1. Set the DNS servers to the ones recommended by your ISP. Apply.
On the iMac, go to Sharing|Internet and choose AirPort Options. Enable WEP encryption (I chose 40-bit with a 5 character password). The default network name is your computer name. I added a postfix to the network name ("-ics") so I can distinguish it from the computer-to-computer network. Apply.
Now start sharing your Built-in Ethernet connection to computers using Airport. As I mentioned previously, this will create the new network and also start a DHCP server.
On the MacBook, set the Airport network to "Use DHCP". Leave all fields blank.
On the MacBook, connect to the newly created network, which should be named "hostname-ics" if you followed my naming advice earlier. Watch the network configuration. Eventually it shows the IP 10.0.2.2.

That's it. You should now have access to your other machines on your private network, but also to Internet.

A cool GMail feature

Ulrik Sandberg — Wed, 03 Jan 2007 16:34:37 +0000

Google Mail will deliver a mail even though the recipient address ends with '+' and some more text, like john.doe+msn@gmail.com for example. This feature can be used to create an unlimited number of mail addresses that can be used for registration purposes. For example, say that the MSN web site requires your email address for you to download some stuff you need. You don't want to give up your real email, since you're afraid you'll be spammed. Yet you need to give a valid email since they might send registration information to it, like the password. The solution is to use john.doe+msn@gmail.com and the mails will still be delivered to the john.doe mailbox. However, you can now quickly identify if the address you gave when you registered at MSN has been used for spam, and create a filter to handle them. GMail filters allow you to perform any or all of the following actions: archive, delete, label, forward, mark with a star.

Note that although this follows the Internet standard for email addresses (RFC 822), some sites simply ignore this and incorrectly disallows addresses with plus signs.