Net::netent − by−name interface to Perl’s built−in getnet*() functions


 use Net::netent qw(:FIELDS);
 getnetbyname("loopback")               or die "bad net";
 printf "%s is %08X\n", $n_name, $n_net;
 use Net::netent;
 $n = getnetbyname("loopback")          or die "bad net";
 { # there's gotta be a better way, eh?
     @bytes = unpack("C4", pack("N", $n−>net));
     shift @bytes while @bytes && $bytes[0] == 0;
 printf "%s is %08X [%d.%d.%d.%d]\n", $n−>name, $n−>net, @bytes;


This module’s default exports override the core getnetbyname() and getnetbyaddr() functions, replacing them with versions that return "Net::netent" objects. This object has methods that return the similarly named structure field name from the C’s netent structure from netdb.h; namely name, aliases, addrtype, and net. The aliases method returns an array reference, the rest scalars.

You may also import all the structure fields directly into your namespace as regular variables using the :FIELDS import tag. (Note that this still overrides your core functions.) Access these fields as variables named with a preceding "n_". Thus, "$net_obj−>name()" corresponds to $n_name if you import the fields. Array references are available as regular array variables, so for example "@{ $net_obj−>aliases() }" would be simply @n_aliases.

The getnet() function is a simple front-end that forwards a numeric argument to getnetbyaddr(), and the rest to getnetbyname().

To access this functionality without the core overrides, pass the "use" an empty import list, and then access function functions with their full qualified names. On the other hand, the built-ins are still available via the "CORE::" pseudo-package.


The getnet() functions do this in the Perl core:

    sv_setiv(sv, (I32)nent−>n_net);

The gethost() functions do this in the Perl core:

    sv_setpvn(sv, hent−>h_addr, len);

That means that the address comes back in binary for the host functions, and as a regular perl integer for the net ones. This seems a bug, but here’s how to deal with it:

 use strict;
 use Socket;
 use Net::netent;
 @ARGV = ('loopback') unless @ARGV;
 my($n, $net);
 for $net ( @ARGV ) {
     unless ($n = getnetbyname($net)) {
        warn "$0: no such net: $net\n";
     printf "\n%s is %s%s\n",
            lc($n−>name) eq lc($net) ? "" : "*really* ",
     print "\taliases are ", join(", ", @{$n−>aliases}), "\n"
                if @{$n−>aliases};
     # this is stupid; first, why is this not in binary?
     # second, why am i going through these convolutions
     # to make it looks right
        my @a = unpack("C4", pack("N", $n−>net));
        shift @a while @a && $a[0] == 0;
        printf "\taddr is %s [%d.%d.%d.%d]\n", $n−>net, @a;
     if ($n = getnetbyaddr($n−>net)) {
        if (lc($n−>name) ne lc($net)) {
            printf "\tThat addr reverses to net %s!\n", $n−>name;
            $net = $n−>name;


While this class is currently implemented using the Class::Struct module to build a struct-like class, you shouldn’t rely upon this.


Tom Christiansen


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.