[olug] Subnet mask

[Phil Brutsche]

Craig Wolf wrote:
> I am having a brain freeze and I cannot for the life of me figure
> this out.  I have been given a range of numbers to use at my job and
> I cannot figure out the subnet mask...Help! The numbers are
> 205.202.101.64-205.202.101.93.

205.202.101.64/27

You can use 205.202.101.65-94 for hosts.  ipsc is a godsend in these 
situations and is available with most distributions:

$ ipsc -a 205.202.101.64/27
Network class:            C
Network mask:             255.255.255.0
Network mask (hex):       FFFFFF00
Network address:          205.202.101.64
Subnet bits:              3
Max subnets:              8
Full subnet mask:         255.255.255.224
Full subnet mask (hex):   FFFFFFE0
Host bits:                5
Addresses per subnet:     32
Bit map:                  nnnnnnnn.nnnnnnnn.nnnnnnnn.ssshhhhh

Subnet 1:                 205.202.101.0    205.202.101.31
Subnet 2:                 205.202.101.32   205.202.101.63
Subnet 3:                 205.202.101.64   205.202.101.95   *
Subnet 4:                 205.202.101.96   205.202.101.127
Subnet 5:                 205.202.101.128  205.202.101.159
Subnet 6:                 205.202.101.160  205.202.101.191
Subnet 7:                 205.202.101.192  205.202.101.223
Subnet 8:                 205.202.101.224  205.202.101.255

IP address:               205.202.101.64
Hexadecimal IP address:   CDCA6540
Host allocation range:    205.202.101.65 - 205.202.101.94
Full subnet mask:         255.255.255.224
Subnet mask:              0.0.0.224
Subnet ID:                0.0.0.64
Network ID:               205.202.101.0
Host ID:                  0.0.0.64

CIDR notation:            205.202.101.0/27
Supernet max:             0
Cisco wildcard:           0.0.0.31
Classful network:         205.202.101.0/24
Route/Mask:               205.202.101.0 / 255.255.255.224
Hexadecimal route/mask:   CDCA6500   / FFFFFFE0

> Class C address from authority.

Irrelevent in the days of CIDR.

 > I know that by thinking about it, it is divided into 4 subnets, right?
 > (30 numbers into 128 is 4 times+).  ugh.  My head hurts today...

-- 

Phil Brutsche
phil at brutsche.us