explanation of the output's command "ifconfig ppp0"

Question

I'm trying to understand the meaning the output of the command:

ifconfig ppp0

Note that I'm using an internet-usb-board and so I'm sure that ppp0 interface is associated to it. In particular I have these questions:

1. What is the field P-t-P:x.x.x.x?
2. Why is the subnet field set to 255.255.255.255?
3. If the inet address is my internet address, why is it different from the internet address computed by a generic web site (I mean http://www.whatismyip.com)?
4. inet = internet? :)
5. At the third line there is the uppercase string UP POINTOPOINT RUNNING NOARP MULTICAST; does it mean that my subnet doesn't support multicast of level 2?

Solution

I'll explain using the output from one of our PPP interfaces, which is a DSL connection:

ppp10     Link encap:Point-to-Point Protocol  
          inet addr:72.66.54.151  P-t-P:10.1.48.1  Mask:255.255.255.255

inet addr is our local IP address (inet indeed stands for Internet). If this isn't the case for you, there must be some NAT somewhere, or other weirdness.

P-t-P is the peer's IP address. Doesn't have to be routable (and this one isn't). The peer no doubt has multiple addresses, one of which is probably routable.

Mask is the netmask. A PPP link is a point-to-point link, so it's /32. The only thing on the other end of the link is the peer. (You may think the entire Internet is there, but it's not, you have to use the peer as a gateway. ip route ls will show the default route via the peer—on your system, our routing is more complicated.)

          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1492  Metric:1

These are the interface flags. It's up, it's a point to point link, it's running (I'm not sure this is any different from up for ppp). It does not use arp (no surprise, it's not Ethernet). It passes multicast traffic. It has a maximum transmission "unit" of 1492 bytes (because this is PPPoE). The metric is 1, which is used by some dynamic routing protocols.

          RX packets:92800 errors:0 dropped:0 overruns:0 frame:0
          TX packets:93810 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3 

Only interesting thing here is the RX/TX count. I suppose errors and dropped are possible, but haven't occured.

          RX bytes:12597954 (12.0 MiB)  TX bytes:14766512 (14.0 MiB)

And this is the total traffic that has passed over the interface since it was created (most likely, pppd started).