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Author Topic: 255.255.255.255 subnet  (Read 3133 times)

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Offline jonnytabpni

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255.255.255.255 subnet
« on: April 10, 2011, 02:39:36 pm »
Hi Everyone,

I've seen some DSL ISPs give out addresses where the subnet is 255.255.255.255 (/32). While I understand that this is a "single host network", there is one thing that is confusing me:

How does the host (or router) which is using this IP reach its gateway, if there are no other hosts in the same subnet?

I am familar with how ARP generally works, however I'm very confused when it comes to /32

Any help would be appreciated

Thanks

JT

Offline Cry Havok

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Re: 255.255.255.255 subnet
« Reply #1 on: April 10, 2011, 04:02:06 pm »
The only option to reach other hosts at that point is a broadcast, which as you've got a point to point link allows you to reach the ISP.
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Offline jimp

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Re: 255.255.255.255 subnet
« Reply #2 on: April 11, 2011, 01:33:08 pm »
As Cry Havok explained, that's the way point-to-point (The first two P's in PPP) links work. You don't really need a gateway per se, just dump the packets on the wire and the other side is the only thing there to pick them up.
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Offline jonnytabpni

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Re: 255.255.255.255 subnet
« Reply #3 on: April 11, 2011, 02:30:02 pm »
Can someone please explain to me the steps the traffic takes? Normally in a normal network with a gateway, I believe that this happens:

The host will put out an ARP who-has request for the gateway IP
The Gateway will respond with its MAC address
The host will send all IP traffic to the gateway (as it knows its MAC address)
The gateway will do the same as above to find its gateway, and so on...

I don't understand how just "putting packets on the wire" can be done; surely don't all packet have to have a dest MAC address?

Thanks

Offline jimp

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Re: 255.255.255.255 subnet
« Reply #4 on: April 11, 2011, 02:40:10 pm »
Not on a point-to-point link. ARP only works for things inside a given subnet.

Though you can dump packets on the wire even on a normal interface, but nothing would pick them up. On a point-to-point link there is no guessing about the intended destination of the packets. Every packet is destined for the other side of the link, be it inbound or outbound.

In routing/cisco terms it's the equivalent of putting an interface as the destination of a route instead of an IP address.
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Offline jonnytabpni

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Re: 255.255.255.255 subnet
« Reply #5 on: April 11, 2011, 03:21:26 pm »
Hi jimp,

I think I understand now. Point to Point links don't use ethernet, hense no ARP, is that correct? And I'm assuming that it's PPP's job to transport the packets to make sure they get to the other side?

Just one last thing is bugging me, if all traffic put on the wire reaches the other end, why does pfsense report a gateway in the gateway field in my PPPoE connection?

Thanks

Offline jimp

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Re: 255.255.255.255 subnet
« Reply #6 on: April 11, 2011, 03:44:49 pm »
A gateway is supplied from the remote end, though it isn't really necessary in the traditional sense. It's useful to ping the other side to be sure the connection is up, and there are other reasons it's there (but I don't recall them offhand)
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