Full Discussion: Subnetting
Special Forums IP Networking Subnetting Post 302993570 by drysdalk on Saturday 11th of March 2017 07:41:05 AM
Hi,

Firstly, from just looking at a single IP, you can't tell how large the network it's a part of is. You need to know the netmask. So for example, if I had an IP of 192.168.1.10, that doesn't tell you anything at all about how large or small the network I'm a part of is.

But if I tell you the netmask - e.g. 192.168.1.10/24 or 192.168.1.10/255.255.255.0 - then you know it's part of a network of 256 addresses, running from 192.168.1.0 to 192.168.1.255.

If on the other hand I'd told you the netmask was 192.168.1.10/25 or 192.168.1.10/255.255.255.128 - then you'd know it was part of a network of 128 addresses going from 192.168.1.0 to 192.168.1.127.

And again, in every network, the very first address and the very last address are not usable for hosts, as they are reserved for other purposes.

You can also get plenty of other netmasks than the old simple Class A, B, C notation tells you. For instance I could just have easily told you my netmask was 192.168.1.10/28, in which case you'd then be able to calculate that the network I was a part of runs from 192.168.1.0 to 192.168.1.15, and conists of 16 IP addresses.

So just from looking at an IP you can't tell anything about the size of the network. And these days, there are far more netmasks and subnet sizes that are far more common than the old Class A/B/C notation.
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whois(1)						      General Commands Manual							  whois(1)

NAME
whois - Internet username directory service SYNOPSIS
whois [-h server] name... The whois command looks up user, host, and organization names in the Network Information Center (NIC) database. OPTIONS
Specifies a whois server other than the default server rs.internic.net. DESCRIPTION
The name argument can specify the following: The name of a registered user The name of a registered Internet host The name of some other entity recognized by the whois server By default, the whois command queries the host rs.internic.net. The operands specified to the whois command are concatenated together (separated by white-space) and presented to the whois the server. The default action, unless directed otherwise with a special name is to do a very broad search, looking for matches to name in all types of records and most fields (such as name, nicknames, host name, and network address) in the database. For more information as to what name operands have special meaning, and how to guide the search, use the special name help. EXAMPLES
To display the NIC database entry for the organization osf.org, enter: whois osf.org Open Software Foundation (OSF-DOM) 11 Cambridge Center Cambridge, MAS 02142 Domain Name: OSF.ORG . . . SEE ALSO
DARPA Internet Request for Comment: NICKNAME/WHOIS (RFC 812) whois(1)

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