Date: Tue, 15 Feb 2000 21:59:36 -0500 From: "Crist J. Clark" <cjc@cc942873-a.ewndsr1.nj.home.com> To: Duke Normandin <01031149@3web.net> Cc: cjclark@home.com, freebsd-questions@FreeBSD.org Subject: Re: Routed and public IPs Message-ID: <20000215215936.E45552@cc942873-a.ewndsr1.nj.home.com> In-Reply-To: <000e01bf77b7$846b2c80$509ec5d1@webserver>; from 01031149@3web.net on Tue, Feb 15, 2000 at 06:18:10AM -0700 References: <000e01bf77b7$846b2c80$509ec5d1@webserver>
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On Tue, Feb 15, 2000 at 06:18:10AM -0700, Duke Normandin wrote: > On Monday, February 14, 2000 11:45 AM Crist J. Clark wrote: > > >On Mon, Feb 14, 2000 at 10:35:19AM -0700, Duke Normandin wrote: > >> Although I'm not involved in this thread, directly or indirectly, > >> I want to thank you for such a great reply. I can't believe you > >> and Ruslan et al -- I'm green with envy. I've saved this thread > >> for future reference, however would you mind defining for me (in > >> laymen's terms) the concept of bridge(4)ing? Something like: > >> "bridging is using a box to bridge a gap between (public & private > >> IPs??) or ?? ". I don't want your info to go to waste on this > >> newbie, so I thought I'd ask. Tia... > > > >A bridge is a network device that operates at layer two of the IP > >stack, the link layer. Hubs and switches are the other most common > >devices that work at layer two. A bridge does not know anything about > >IP addresses, and most often, it simply forwards _all_ packets it > >receives on one interface to the other. However, it is possible to run > >a filter on the bridge, as was the whole point of the thread you are > >following. > > > >I personally have only used a simple bridge that passes all > >packets. Some users want two computers (running different OSes) in > >their offices. There is only one RJ-45 connection coming into the > >room. Rather than give them each a hub, one computer gets an extra NIC > >and bridges for the other. As far as the second computer is concerned, > >its on the same LAN. > > > Thanks....to your reply and a tutorial I found on the net, I now > understand that a bridge is an external, physical device. Quoting > this tutorial: > > "In contrast to hubs, which are physical-level devices, bridges > operate on Ethernet frames and thus are layer-2 devices. In fact, > bridges are full-fledged packet switches that forward and filter > frames using the LAN destination addresses. When a frame comes into > a bridge interface, the bridge does not just copy the frame onto all > of the other interfaces. Instead, the bridge examines the destination > address of the frame and attempts to forward the frame on the > interface that leads to the destination." > > However, I'm confused with your term "physical-level device" as > opposed to "link-level (layer-2) devices. Are not both physical > devices? Do not both operate on Ethernet frames? I now understand > that a bridge will "examine" an Ethernet frame, as opposed to a hub, > i.e., "in-one-ear, out-the-other" ;^). Thanks for your time. _My_ term "physical-level?" The physical layer is the bottom layer of the IP stack. The layer where they push electrons around and all of that fun RF stuff. As someone brought up in the thread, the quote above refers to a "hub" as a physical-layer device. That is a device that simply gets some electronic signal in and then sends the same signal out. Unfortunately, the terminology is not that clean. For example, the description of a bridge above seems a lot more like the description of a switch. And if a hub is a layer one and switch layer two, what the heck is the very common item called a "switched hub?" But getting back to the original theme, when we talk about bridging in FreeBSD networking, what it boils down to is passing packets bewteen interfaces on a host without altering the packets' hardware addresses. -- Crist J. Clark cjclark@home.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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