Digital Watchdog Starlight D1363D Información técnica descargar pdf (Pagina 97)

these numbers are derived. Additionally, these unique decimal numbers can also be used by your web

browser in the URL bar. But the reason we use IP addresses in dotted decimal notation is to help

network admins better “see” the contiguous parts of the network (i.e., which IPs are neighbors and

which aren't). It's rather obvious looking at an IP address which IPs are neighbors because on a Class C

network, if the first three octets are the same, then obviously, IPs different only in the last octet are

neighbors. That is 192.168.1.1 and 192.168.1.254 are neighbors on their Class C network because the

first three octets are the same. This means that these two IPs can talk to each other directly without

need of a gateway. IPs that are not neighbors (not on the same subnet), cannot talk to each other

directly, but need to go through a gateway. The two Class C addresses 192.168.1.1 and 192.168.2.1 are

not neighbors and require a gateway in order to communicate.

NETMASK

This brings us to the concept of a netmask. A netmask defines the network. That is, the netmask

defines which IPs are neighbors and can talk directly to each other, and which IPs are not neighbors and

therefore must use a gateway to access a non-neighbor IP.

Under the obsolete Classful concept, netmasks were one of 255.255.255.0, 255.255.0.0, or 255.0.0.0.

The 255's indicated the network portion of the IP, and the zeros indicated the host (neighbor) portion of

the IP. If the network octets matched (192.168.1.x), then the hosts were neighbors and could talk

directly to each other.

Note that under the classful concept of a network, netmask numbers were either 255 or 0. That is, the

binary representation was an octet full of ones or zeros. But to implement CIDR, we must now allow

for other representations. The network part of an IP is still represented in binary by ones and the host

portion by zeros, but the ones and zeros no longer stop neatly at the decimal places. This means that

we can have numbers other than 255 and zero, but that the binary representation will still have a

definitive dividing point. That is, when the netmask is translated from decimal into binary notation,

under VLSM we will have binary ones to the left, then zeros to the right. We will have no binary ones

to the right of the first zero. (Note: CIDR does not carry this limitation, but a non-VLSM netmask is an

academic exercise only – unless you are either crazy or masochistic.).

What the above paragraph means is that a valid VLSM netmask will contain a 255 followed by any one

of 255, 254, 252, 248, 240, 224, 192, 128, or 0, and if that number is not 255, then any subsequent

numbers must be zero. This is because if you translate the aforementioned decimal numbers to binary,

you can clearly see ones to the left, zeros to the right:

255: 11111111

254: 11111110

252: 11111100

248: 11111000

240: 11110000

224: 11100000

192: 11000000

128: 10000000

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Digital Watchdog Starlight D1363D Información técnica Pagina 97