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copied!<p>an ip address is really just an array of <code>unsigned char</code>.</p> <pre><code>uchar ip[] = {127,0,0,1}; </code></pre> <p>Is a fine representation for a loopback address. But an array of four bytes, and a int really aren't that large; with one exception <strong>endiannes</strong>! So suppose I create the <code>int</code> which represents that ip. A naive approach might be:</p> <pre><code> int ip = (127<<24)|(0<<16)|(0<<8)|(1) </code></pre> <p>Of course, on <em>little endian</em> machines, like the x86 and arm it doing this:</p> <pre><code>char *char_ip = (void*)&ip; </code></pre> <p>and iterating over that aray would yield:</p> <pre><code>1, 0, 0, 127 </code></pre> <p>But on a <em>big endian</em> machine, like a PowerPC, or a SPARC, we would have what we expect, </p> <pre><code>127, 0, 0, 1 </code></pre> <p>Big endian is also known as "network byte order" which is what the n in htonl stands for: "host to network long". These functions are frequently used when reading or writing integers over the network. Suppose a server wants to send a client some number:</p> <pre><code>uint32_t important = htonl(42); write(client, &important, sizeof important); </code></pre> <p>Then, to read it, the client goes:</p> <pre><code>uint32_t important; read(server, &important, sizeof important); important = ntohl(important); </code></pre> <p>The reason your ip address were flipped was because ip addresses are expected to be in network byte order, but yours were little endian instead. <code>htonl</code> on an <code>int</code>-type ip will flip it for you.</p>

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