Q My Canon printer and iMac are connected by Ethernet to my Netgear router, which also connects wirelessly to my MacBook. Why can the MacBook only see the printer (and print to it) when the iMac is running?
A You have a misconfiguration that probably results from using a mixture of DHCP and statically-assigned IP addresses on your small network. These can be very difficult to diagnose, particularly if you do not tackle them in a systematic way.
The first thing to check is the IP address of each of the four devices, including your router, when both iMac and MacBook are running and connected. Remember that you are interested here in the local (LAN) address of the router, not its Internet (WAN) address. All four need to be on the same sub-net, such as 192.168.1.x.
What often happens when configuring laptops is that they are left to get an address by DHCP, which may end up on a different sub-net; printers can also sometimes be a pain, either obtaining their address by DHCP or using an internally-determined fixed address that can be tricky to change.
The simplest solution is to assign each device a static (fixed) IP address on the same sub-net, with (for example) 192.168.1.253 for the router, 192.168.1.1 for the printer, and so on. To check that these are all working correctly, use Network Utility’s Ping pane to test each in turn. If you cannot achieve that reliably, then step through the configuration of your router using its documentation, for static IP addresses without the use of DHCP. Another possibility to consider is that your Ethernet cabling may have a fault; if you have spare cables try swapping those in.
Updated from the original, which was first published in MacUser volume 26 issue 10, 2010.