Digital Watchdog Starlight D1363D Información técnica descargar pdf (Pagina 98)

0: 00000000

So we can have a netmask that looks like: 255.255.255.224, or 255.255.248.0, or 255.240.0.0. But we

cannot have 255.0.240.0 or any decimal number not listed above (e.g., 255.255.255.220).

VLSM Notation

Along with the use of VLSM and netmasks composed of other than 0 and 255, a new convention was

designed to more easily denote these new VLSM subnets. This VLSM notation is nothing more

complicated than counting the number of ones in the netmask and showing how many there are. This

notation can be used in lieu of a netmask as follows:

if we have a netmask of 255.255.255.0, we can see that this is 3 octets of 8 ones in binary. So an IP

address can be said to be 192.168.1.1 netmask 255.255.255.0 or 192.168.1.1/24. The /24 tells you how

many ones are in the network portion of the netmask. So a netmask of 255.255.255.240 is the same as

a /28. The /28 is often called a suffix because it is suffixed to the IP address in lieu of the netmask.

Since there are a total of 32 bits in an IPv4 IP address, we can see that there are 4 zeros (32 – 28 = 4).

Now it just so happens that 4 ones in binary (1 1 1 1) == 16. It's not a coincidence that a /28 network

(netmask 255.255.255.240) will give you exactly 16 IPs in that subnet. Subtract one IP for the network,

one for the broadcast address and you have 14 host IPs. You'll need to use one IP for your gateway

router, so now have 13 IPs to use for your hosts (clients).

This means that a netmask 255.255.255.248 or /29 suffix has 3 binary places or 8 IPs, and a netmask of

255.255.255.224 or /27 has 5 binary places or 32 IPs.

Remember I told you all you had to be able to do was multiply and divide by 2, 4, 8, 16, 32, 64, or 128

to subnet? The reason should start becoming clear:

Netmask Last octet in binary suffix # IPs

255.255.255.0 == 00000000 == /24 == 256

255.255.255.128 == 10000000 == /25 == 128

255.255.255.192 == 11000000 == /26 == 64

255.255.255.224 == 11100000 == /27 == 32

255.255.255.240 == 11110000 == /28 == 16

255.255.255.248 == 11111000 == /29 == 8

255.255.255.252 == 11111100 == /30 == 4

255.255.255.254 == 11111110 == /31 == 2 (unusable)

255.255.255.255 == 11111111 == /32 == 1

Subnetting

To subnet, we will just take a netblock we have and carve it up. We can carve up a full “Class C” into 2

/25 networks, or 4 /26 networks, or 8 /27 networks, or 16 /28 networks, or 32 /29 networks. But we

don't have to restrict ourselves to dividing them up in just one way. We may need two /27 subnets, 4

/28 subnets, and whatever is left as one contiguous block. So we begin.

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Digital Watchdog Starlight D1363D Información técnica Pagina 98