Set up bridge between Aiport and Build-in-Ethernet

I need to set up a bridge, so that everything that comes through the Airport or the Build-in-Ethernet will be bridged through my Powerbook.
That build in NAT-funktion in System Preferences-Sharing-Internet does not help me, because NAT is not able to transport broadcasts...

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What. What are you trying to do? Bridged through to what?
birdofprayAuthor Commented:
I have to connect a PC to my Build-in-Ethernet so that it can get a lease from my DHCP-Server but there is no cable in that room, just a wireless network.
Ok, so what's wrong with getting a DHCP address from the airport base station?
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I think that the laptop *is* the base station.  If I read the question correctly, the actual network is wired, but the PC only has a wireless NIC; is that corrct?
birdofprayAuthor Commented:
That's correct
birdofprayAuthor Commented:
Or to ask more simple:

Has the Darwin-Kernel an option to activate bridging in general, and can I activate it with a simple ipfw command?
In the sharing control panel (internet tab) enable connection sharing for the interface you want to share.
birdofprayAuthor Commented:
With this control tab I activate a NAT, but I need ethernet bridgeing
It "shares" the connection. That's bridging interfaces.
No, a bridge (in this context) shares one IP address across two devices, effectively merging them into one device.  This allows broadcast packets (e. g. DHCP requests and replies) to pass through a device, which NAT does not.

Unfortunately, I do now know how to do this in OS 10.

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Errr...I've never heard *that* definition of a bridge.
Windows XP offers this function in its 'Network Connections' control panel.  The actual definition for this is a 'brouter' (a unification of bridge and router), wihch is basically a router that also passes along broadcast traffic.

In-depth information on brouters can be found here:,,sid7_gci211707,00.html
Same thing OS X does via connection sharing.
No, that's NAT, not bridging.  You have a brouter/bridge when both interfaces have the same IP address.
A bridge is when you "bridge" traffic across network interfaces.
Correct.  This is different from NAT in many fundamental ways.

 NAT (Network Address Translation) is the translation of an Internet Protocol address (IP address) used within one network to a different IP address known within another network. One network is designated the inside network and the other is the outside. Typically, a company maps its local inside network addresses to one or more global outside IP addresses and unmaps the global IP addresses on incoming packets back into local IP addresses. This helps ensure security since each outgoing or incoming request must go through a translation process that also offers the opportunity to qualify or authenticate the request or match it to a previous request. NAT also conserves on the number of global IP addresses that a company needs and it lets the company use a single IP address in its communication with the world.

NAT is included as part of a router and is often part of a corporate firewall. Network administrators create a NAT table that does the global-to-local and local-to-global IP address mapping. NAT can also be used in conjunction with policy routing. NAT can be statically defined or it can be set up to dynamically translate from and to a pool of IP addresses. Cisco's version of NAT lets an administrator create tables that map:

    * A local IP address to one global IP address statically
    * A local IP address to any of a rotating pool of global IP addresses that a company may have
    * A local IP address plus a particular TCP port to a global IP address or one in a pool of them
    * A global IP address to any of a pool of local IP addresses on a round-robin basis

NAT is described in general terms in RFC 1631. which discusses NAT's relationship to Classless Interdomain Routing (CIDR) as a way to reduce the IP address depletion problem. NAT reduces the need for a large amount of publicly known IP addresses by creating a separation between publicly known and privately known IP addresses. CIDR aggregates publicly known IP addresses into blocks so that fewer IP addresses are wasted. In the end, both extend the use of IPv4 IP addresses for a few more years before IPv6 is generally supported.

All that notwithstanding, birdofprey, have you tried setting buth interfaces to the same IP address in the Network panel of System Preferences?
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