Question

Why is the IPv4’s decimal value different with inet_pton and inet_addr (1734763876) than what you get if you use these 2 websites (1684366951) ?

struct sockaddr_in sin;
inet_pton(AF_INET, "100.101.102.103", &(sin.sin_addr));
printf("%i\n%i\n", inet_addr("100.101.102.103"), sin.sin_addr);

• http://www.allredroster.com/iptodec.htm
• http://www.ipaddresslocation.org/convertip.php

Answer 1

Endianness – they have the four bytes in opposite orders:

1734763876 = 0x67 66 65 64
1684366951 = 0x64 65 66 67

The value you’ll need to use for URLs etc. is the one in ‘Network’ order, Most-Significant-Byte first. Use htonl() (host-to-network-long) to convert the value, i.e.

printf("%i\n%i\n", htonl(inet_addr("100.101.102.103")), htonl(sin.sin_addr));

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caf points out below that I probably have this backwards: the issue is really that you need to convert the network-order data from the socket functions back into host-order for display, i.e.

printf("%i\n%i\n", ntohl(inet_addr("100.101.102.103")), ntohl(sin.sin_addr));