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MBOX® USER MANUAL
Subnet Masking Basics
For pretty much all communication to and from Mbox, each octet in an interface’s subnet mask will be set to either 0 or
255. Values other than 0 or 255 can be used for more complicated subnetting, but are typically not needed with Mbox
and won’t be discussed here.
In order to understand how an interface’s subnet mask works, it needs to be viewed together with the interface’s IP
address. If an octet in the subnet mask has a value of 255, then the corresponding octet in the interface’s IP address
is used to designate the subnet that the interface belongs to. If an octet in the subnet mask has a value of 0, the
corresponding octet in the IP address is used as part of the interface’s unique identier on that subnet. Within the
structure of sub-networks and unique identiers, it is possible to have multiple interfaces on the same subnet, each
being uniquely identiable. Data can be sent to all interfaces in the subnet by broadcasting, to groups by multicasting,
or to individual members by unicasting.
How the IP Address and Subnet Mask Work Together
For a device with the IP address of 192.168.0.123 and a subnet mask of 255.255.0.0, the rst two octets of the IP
address “192.168” are used to specify the sub-network, and the last two octets “0.123” specify the device’s unique
identier within that network.
Depending on the number of octets with a value of 255 in the subnet mask, different descriptions may be given to the
interface. A subnet mask of 255.0.0.0 leads to a description of “/8” for the interface. Combined with the interface’s IP
address, this information would be written as “192.168.0.123/8”. A subnet mask of 255.255.0.0 gets a description of
“/16” and 255.255.255.0 a description of “/24”.
Note: There is no such thing as a /32 subnet mask, as the last octet in an interface’s IP address must always be used as
a unique identier.
Routing Ethernet Communications
+ Full Broadcast - data packets that are sent to the full broadcast destination IP address (255.255.255.255) will be
received by all devices on the physical network provided the packets are not blocked by a switch or router between
devices. Full broadcast is akin to shouting in a crowded room. Everyone can hear the yelling, but must decide
whether to listen or not. Full broadcast is simple and guarantees for the most part that every device on the network
can get the data. However, it places a lot of data onto the network, thus using up more of the available bandwidth.
Another downside is that receiving devices may need to spend more time than is desirable to lter out data that they
don’t need to listen to.
+ Subnet Broadcast - packets that use a subnet broadcast IP address (x.255.255.255, x.x.255.255, or x.x.x.255)
will only be received by interfaces on the matching subnet - i.e., those with a subnet mask that places them into
the same subnet. This is one step down from shouting, as subnet ltering allows receiving devices to immediately
discard any subnet broadcast data that isn’t destined for their subnet.
Network Unique ID
Subnet Mask: 255 255 000 000
IP Address: 192 168 000 123
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