For all those who don’t like to remember the details I have included my quick reference to IPv4. It’s not full fledged but can offer some pointing in the right direction. IPv4 is the fourth iteration of the Internet Protocol (IP) and is the most widely deployed. If you want some ugly history details you can always refer to IETF RFC (791). Otherwise just know IPv4 is a data oriented protocol to be used on packet switched internet works such as ethernet.
The following shows Version 4 IP addressing consisting of 32-bit binary number of four octets (dotted decimal notation)
Classful Addresses
Class A (24 Bit) 1-126.x.x.x
Class B (20 Bit) 128-191.x.x.x
Class C (16 Bit) 192-223.x.x.x
Class D Multicast 224-239.x.x.x
Internal Loopback 127
Private IP Address Ranges
Class A (24 Bit) 10.0.0.0-10.255.255.255 CIDR Mask /8 Decimal Mask 255.0.0.0
Class B (20 Bit) 172.16.0.0-172.31.255.255 CIDR Mask /12 Decimal Mask 255.240.0.0
Class C (16 Bit) 192.168.0.0-192.168.255.255 CIDR Mask /16 Decimal Mask 255.255.0.0
/24 IP Bit Mask
Dotted Decimal Mask: 255.255.255.0
Effective Hosts: 254
Effective Subnets: 1
Usable IP's: 254
/25 IP Bit Mask
Dotted Decimal Mask: 255.255.255.128
Effective Hosts: 252
Effective Subnets: 2
Usable IP's: 126
/26 IP Bit Mask
Dotted Decimal Mask: 255.255.255.192
Effective Hosts: 248
Effective Subnets: 4
Usable IP's: 62
/27 IP Bit Mask
Dotted Decimal Mask: 255.255.255.224
Effective Hosts: 240
Effective Subnets: 8
Usable IP's: 30
/28 IP Bit Mask
Dotted Decimal Mask: 255.255.255.240
Effective Hosts: 224
Effective Subnets: 16
Usable IP's: 14
/29 IP Bit Mask
Dotted Decimal Mask: 255.255.255.248
Effective Hosts: 192
Effective Subnets: 32
Usable IP's: 6
/30 IP Bit Mask
Dotted Decimal Mask: 255.255.255.252
Effective Hosts: 128
Effective Subnets: 64
Usable IP's: 2