Oh. Silly me. You have a problem there: unpack consumes its input in byte chunks. Even if you specify b4, it will consume a full byte. (If you specificy b10, it will eat two full bytes.) Note also that b8 will return something of the form 00011111, not 31 - for that purpose you need C (unsigned character). For in-byte bit fiddling, you need vec: 
my ($ver_hlen, $tos, $len) = unpack "C2n", $foo;
my ($ver, $hlen) = (vec($ver_hlen, 4, 0), vec($ver_hlen, 4, 1));

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or more compactly my ($ver, $hlen) = map( vec($ver_hlen, 4, $_), (0 .. 1) ); See perldoc -f vec for details.

Makeshifts last the longest.

In reply to Re^3: IP Header by Aristotle
in thread IP Header by Anonymous Monk

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