[WinPcap-users] Retrieving a pointer to a UDP or TCP header

[Mario Hollibaugh]

   In the WinPcap v 3.1 beta 4 documentation, tutorial section 
"Interpreting the packets" there is some code as an example.  If you 
look at that code, on line 203 he "retrieves the position of the udp 
header".  He uses the following line of code:

ip_len = (ih->ver_ihl & 0xf) * 4;
uh = (udp_header *) ((u_char*)ih + ip_len);
where the declarations have been made as follows (which u can also, of 
course, see at the top of the code)

int ip_len
ip_header *ih;
udp_header *uh;
  
I don't understand how this line of code is giving him the location of 
the udp header.  In the first line where he says (ih->ver_ihl & 0xf) * 
4, why is he first using that bit-wise and operator with the hex value 
that follows?  And why is he multiplying all of that by 4?  If you 
wanted to know the length of the IP header why wouldn't you just take 
the 4-bit value that is passed in an IP header as the IP header length 
field?

cheers,
Mario

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