原因は安価なルーターの NAT の仕様。ルーターにはちゃんとした a full cone NAT router が必須のようです。

iChat > Video > Connection Doctor

Show: Network Status

Router type: Port Restricted
This computer's network setup includes
one or more devices
that are not fully compatible
with audio and video chatting.
iChat Help

Solving problems
Other problems
If your router is causing connection problems

When the Network Status pane of Connection Doctor indicates that your computer uses a router of type Restricted, Port Restricted, Symmetric, or Unknown, the approach to fixing connectivity problems depends on your network setup.

Try these suggestions:

If you're using an office network with simple networking policies, consult your network administrator.

If you’re using a home network with a DSL/Cable modem and a network address translation (NAT) router, instead of having address translation performed by the modem, install a full cone NAT router.

If you’re using a home network with a DSL/Cable modem that has a built-in NAT router, ask your Internet Service Provider (ISP) about a replacement modem that’s compatible with a full cone router.

If you're using a home network and use an ISP that uses NAT and provides only private addresses, consider switching to a different ISP or negotiate more open NAT support from your existing ISP.

To learn more about NAT routers, choose Video > Connection Doctor, choose Network Status from the Show pop-up menu, and then click the Help button (looks like a question mark).

Related Topics:

If you experience a connection failure or diminished audio/video quality

topic: macosx
first posted: 2012-02-22 05:08:05
last modified: 2012-02-28 00:13:19