ipcalc is a DHTML application that calculates all relevant aspects of a given IP address or network, given an IP address and a netmask specification. It's a pretty straightforward ripoff of Keith Owens' original Perl script, but all errors are my fault alone.
Several different items are displayed as a result of a query. Our example for purposes of explanation will use an IP from a fairly standard class C "private" network, 192.168.1.1/24. ipcalc is not limited to outdated conceptions of class, however — any RFC 1817-style VLSM will work, as well. New! Support for input of IP in hexadecimal format; just append the netmask at the end and separate them with a slash as before.
| IP address | 192 . 168 . 1 . 1 / 24 | 192.168.1.1/24 |
| Netmask bits | 11111111 11111111 11111111 00000000 | |
| Netmask bytes | 255 . 255 . 255 . 0 | 255.255.255.0 |
| Address bits | 11000000 10101000 00000001 00000001 | |
| Network | 192 . 168 . 1 . 0 | 192.168.1.0 |
| Broadcast | 192 . 168 . 1 . 255 | 192.168.1.255 |
| First Host | 192 . 168 . 1 . 1 | 192.168.1.1 |
| Last Host | 192 . 168 . 1 . 254 | 192.168.1.254 |
| Total Hosts | 254 | |
| PTR | 1.1.168.192.in-addr.arpa | |
| IP Address (hex) | 0xc0a80101 |
The first line in the output contains the original IP address you supplied, with subnet mask, formatted for easy comparison with the following.
The second line contains a representation of the netmask as a binary number.
The third line contains a representation of the netmask as a "dotted quad", or the netmask expressed as four byte values separated by periods. This is the most common form in which IP addresses are seen in user space.
The fourth line contains a representation of the IP address as a binary number.
The fifth line contains the network number based on the IP and netmask you supplied.
The sixth line contains the broadcast address (used for traffic that all hosts need to receive, such as ARP/RARP messages).
The seventh line contains the IP address in dotted quad format of the first host on the network named above.
The eighth line contains the IP address in dotted quad format of the last host on the network named above.
The ninth line gives the number of unique hosts possible on the network.
The tenth line gives the PTR, or "in-addr.arpa" record for the host or network named in the "IP Address" field.
The final line gives a hexadecimal representation of the IP address listed in line one.
Several of the fields (particularly those that use a preformatted dotted quad presentation) are also provided without the intervening spaces, for ease of cut-and-paste.
ipcalc was originally written by Keith Owens, as a Perl script. For kicks, I ported it to Javascript, with help from Google. I used and somewhat modified a Javascript printf() library that I found on what appears to be Shigenobu AOKI's Web site (I don't read Japanese, and the code wasn't in English). Thanks to Brent Verner and Greg Knauss for testing.