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In case of PPP, are we allowed to use any IP address including broadcast & network id as the source address?

For example:

192.168.1.0, 192.168.1.255?

One of the customer has configured a DHCP pool for a PPP as 192.168.0.1 to 192.168.1.254. (instead of creating two separate range: 192.168.0.1~192.168.0.254, 192.168.1.1~192.168.1.254). When the DHCP issues 192.168.1.0 to the client, the PPP still works. It made me wonder whether they are legal to use in PPP (being a /32)? I thought being a network ID we should not use this as source address. Is it because PPP uses /32 and hence it is no longer network id?

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2 Answers 2

These are two different issues mentioned here:

  • The usage of a 32 bit netmask over a PPP link:

Imagine each PPP connection from a remote host to the PPP server as a separate cable which is a small, 2 node network of it's own, isolated from everything else, no gateways, no subnets, everything is routed on-link.

PPP is commonly used with a 32 bit netmask because there are only two points on a given PPP link and thus no routing needs to take place on the link, all traffic from one end will always reach the other end no matter the address.

Routing will take place outside the PPP link, the PPP server will usually have routes in place for every PPP interface (connection) that has been established, so for example, let's say the corporate mail server is replying to a remote PPP client.

The packet will go from the mail server to the PPP aggregator server (PPPSRV1), the destination address is 192.168.1.10 (PPPHOST1); PPPSRV1 finds the most specific route in it's routing table which might look something like:

Target: 192.168.1.10/32, Gateway: 0.0.0.0 (on-link), Device/Interface: ppp8

PPPSRV1 will route the packet from the ethernet port to the PPP connection via the interface ppp8 where it will reach PPPHOST1.

RFC3021 is not directly related, but it explains some related concepts.

My point is, PPP doesn't have to use a 32 bit netmask.

  • Confusion regarding network ID and broadcast addresses on the 192.168.0.0/23 network:

Like joequerty said, a 23 bit netmask was used which means that the network ID and broadcast addresses are 192.168.0.0 and 192.168.1.255 respectivly, anything in between can be assigned to hosts. This is the most straight forward calculation - not covered by the network bits? off the hosts you go.

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If they're using a /23 netmask (255.255.254.0) then 192.168.1.0 is a valid host address. In this case 192.168.0.0 is the network address, 192.168.1.255 is the broadcast address, and the valid host addresses are 192.168.0.1 through 192.168.1.254.

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That particular piece of software where he creates the IP lease pool, does not have a provision to enter subnet mask (just a plain start IP and end IP). Thus I was assuming, if a subnet mask is not specified, it will consider it as classful addressing and hence my doubt. Being a Class-C IP range, if we use /23, it will be considered as Supernetting right? Is Supernetting is meant only for route summarization or is it also allowed to be used in general purpose DHCP range? –  mks Feb 22 '12 at 6:51
    
No, supernetting is a different topic. Using a /23 would be considered classless, VLSM, or CIDR. If the software doesn't allow you to set the subnet mask then I'll assume that it supports classful addressing only (as you stated in your comment) and so my answer probably doesn't apply. –  joeqwerty Feb 22 '12 at 11:53
    
After a bit of testing, finally noticed that it is Windows bug: support.microsoft.com/kb/281579 The same address 192.168.0.255 (Under Supernetting 192.168.0.0/23) works perfectly fine with Linux. –  mks Feb 23 '12 at 4:48

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