Mac OS: Connecting to the Internet and sharing files locally at the same time

This document explains how to set up your computers so that they may connect to each other (your "local network") and the Internet at the same time. This allows you to use File Sharing locally while web browsing, for example. The same methods apply for any other local TCP/IP activity.

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When you set up file sharing between computers and try connect to the Internet, you may see the following symptoms:

  1. A computer can share files or connect to the Internet, but it cannot do both at the same time.
  2. A Mac OS 9 computer can see a Mac OS X computer but cannot connect to it. The modem on the Mac OS 9 computer may attempt to dial up when you attempt to connect to the Mac OS X computer.
  3. A Mac OS X computer may experience significantly increased startup time (due to inappropriate network settings).

These symptoms may be due to lack of a router or equivalent alternative on your local network. A common scenario for this is when you add a second computer to your home, connect the two via Ethernet for file sharing, but still use a 56 kbit/s modem on each computer to connect them to the Internet. Though this document uses File Sharing as an example, it could also apply to other TCP/IP services that you use on your local area network (LAN).

The ideal way to resolve this situation is to add a router to your network, but there are alternate solutions discussed below. In a home or other small network, the router connects to the Internet and then shares the connection with your computers. The router holds the public Internet Protocol (IP) address that is issued by your Internet service provider. Your computers get "private" IP addresses for communicating with the router and with each other. The advantage of a router is that your computers use the same IP configuration for communicating with each other and with the outside world. All of the symptoms above are generated by mismanagement of the dual IP configurations that are necessary in the absence of a router.

What kind of router to get?

Routers vary greatly in cost, features, and capacity. You most likely only need a router of the variety designed for home use. These "table top" routers are much smaller and less expensive than their commercial rack-mounted counterparts. If you connect to the Internet with a dial-up modem, be sure to look only at routers that come with a built-in modem, such as the Apple AirPort Base Station. The connectivity combinations you might find in a router are:

  • Ethernet only
  • Ethernet and 56 kbit/s modem (needed for dial-up)
  • Ethernet and 802.11 wireless Ethernet
  • Ethernet, 56 kbit/s modem, and 802.11 wireless Ethernet

The Apple AirPort Base Station (Dual Ethernet) is an example of the latter, combining service for both wired and wireless Ethernet clients with the ability to connect to the Internet via dial-up or broadband. Even if you do not plan on adding any wireless computers to your network immediately, you can still use the AirPort Base Station (Dual Ethernet) for its wired routing functions.

You would then follow the instructions included with your router for assigning IP addresses to the client computers. This may be done automatically in most cases by setting the computers to "configure via DHCP". In Mac OS 9, this is done in the TCP/IP control panel. In Mac OS X, this is done in the Network pane of System Preferences. You may still need to manually type the DNS server addresses in the TCP/IP control panel or Network preference pane, depending on your router and how you set it up. DNS server addresses are provided by your Internet service provider. If you get an AirPort Base Station, you may use the AirPort Setup Assistant. Additional information is available from these .

Alternate solutions

If you do not want to get a router, use these solutions. These alternate solutions assume that you know to respect the limits of your Internet service provider (ISP). Be sure to consider these factors:

  • If all of your computers have dial-up modems, you may only connect one computer to the Internet per phone line (and possibly per ISP account) at any given time.
  • If you have a broadband (cable, DSL, satellite, or other) connection, only one computer at a time will be able to connect to the broadband modem until you request from your ISP an additional IP address for each computer (fees may apply).
  • Some broadband modems require that you turn off or unplug them before they will recognize a new computer.
  • The "modem" provided by some broadband ISPs may also be a router. Check with your ISP if you are not sure.

Mac OS X

Mac OS X can hold two or more IP addresses at the same time and automatically switch between them. But for optimum performance, you need to set them up properly.

The network interface, or "port," (Ethernet, AirPort, or Modem) that connects to the Internet needs to be first in your port configurations list. This means it is the most preferred port, the one that Mac OS X looks to first when starting up and when connecting to Internet sites. The port (Ethernet or AirPort) that you use to connect to for local sharing should be second. Follow these steps:

  1. Choose System Preferences from the Apple menu.
  2. Choose Network from the View menu.
  3. Choose the port you use to physically connect to the Internet from the Show pop-up menu. (Prior to Mac OS X 10.1, the Show menu was named "Configure".)
  4. Click the TCP/IP tab.
  5. This pane should already be set up as directed by your Internet service provider. If it is not, make corrections. For more specific setup information, see Solutions for connecting to the Internet, setting up a small network, and troubleshooting
  6. Choose Network Port Configurations from the Show pop-up menu. (This is the language for Mac OS X 10.2 or later. In 10.1 versions, you would choose"Active Network Ports". In 10.0 versions, you would choose "Advanced".)
  7. If the port you chose in Step 3 is not at the top of the list, drag it to the top. Users of portable computers may need to prioritize different network interfaces for each place that they use their computers. You may, for example, prefer Internal Modem when at home but AirPort when at work or school. If this is your case, you may wish to read more about the Location menu.
  8. Locate the port you use for local File Sharing.
  9. If this port is not second in the port configurations list, drag it to the second position.
  10. Choose this port from the Show pop-up menu.
  11. Click the TCP/IP tab.
  12. Choose Manually from the Configure pop-up menu.
  13. Enter these settings: Subnet mask: IP address: 10.0.1.x The "x" in the address 10.0.1.x represents any number between 1 and 255, for example "". Just be sure that each computer has a different address. Leave the router and other fields blank.
  14. Click Apply Now.

Mac OS 9

If you do not have any Mac OS X computers on your network, you only need to open the File Sharing control panel and make sure that the checkbox for the "Enable File Sharing clients to connect over TCP/IP" feature is not selected. This will allow the connection to occur over AppleTalk, without interfering with your IP configuration.

If you do have Mac OS X computers on your network, TCP/IP is required for file sharing with a Mac OS X computer. This creates a need for your Mac OS 9 computer to hold two IP addresses, as Mac OS X can. The TCP/IP control panel in Mac OS 9 does not offer this option, but the underlying Open Transport software is capable of doing so. To take advantage of this feature in Open Transport, you may use third-party routing software such as IPNetrouter. Using the third-party routing software, configure the Mac OS 9 computer hold both public and private IP configurations. An example would be to set it to connect to the Internet using 56 kbit/s modem but also to hold a 10.0.1.x address. The "x" in the address 10.0.1.x represents any number between 1 and 255, for example "". Just be sure that each computer has a different address.

Using a software router has the additional advantage of being able to connect two or more computers simultaneously via one dial-up account, just you can with an independent hardware router. See the documentation included with any router software.

Location manager

A final alternative available to Mac OS 9 users is Location Manager. This does not allow you to share and connect to the Internet at the same time, but it does allow you to switch quickly and painlessly between two different IP configurations. Create one location named "Internet" that is set up for your public connection, and a second location named "File Sharing" that is set up for your local 10.0.1.x address. Use the Location Manager portion of the Control Strip to switch between them. For setup information, Choose Mac Help from your computer's Help menu and search on "Location Manager".

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