Subnetting Example-Transparent Subnetting - IBM 5500 Operational, Installation, And Maintenance Manual

Subnetting Example-Transparent Subnetting

IBM recommends that when you install the 5500 Control Unit into an existing
network, you should subnet your existing address space to avoid problems.
Subnetting is accomplished by partitioning an existing address space and
using each set of addresses for different sets of devices. Partitioning can be
accomplished by using a subnet mask that is more restrictive for the twinax
workstations attached to the 5500 Control Unit than for the LAN to which the
5500 Control Unit is also attached. A simple example follows:
• Original network uses subnet mask 255.255.255.0 and is assigned
addresses 100.100.100.0 through 100.100.100.255 (see Figure 14 on page
44).
• Desire to split this address space so that 64 addresses are reserved for use
by the twinax workstations attached to the IBM 5500 Control Unit and
192 addresses are left for other LAN devices.
• Twinax workstations use subnet mask 255.255.255.192 and can take any
of four contiguous 64 address ranges between 100.100.100.0 and
100.100.100.255.
In this example the twinax workstations use the upper end of the available
range of addresses, in this case 100.100.100.192 through
100.100.100.255. It is also critically important that none of the addresses
in the range reserved for the twinax workstations be assigned to any
device anywhere else on the network.
• Since 3 of these addresses are consumed by the TCP/IP network
architecture (you should not use the first, second or last address in the
range), there are 61 addresses left for use by twinax workstations attached
to the IBM 5500 Control Unit. (100.100.100.194 through
100.100.100.254).
• No upstream router table changes are required since the IBM 5500
Control Unit and the total address space was already properly routed to
the existing LAN.
• No subnet mask changes are required on devices attached to the LAN
(any other time you subnet a network all of the subnet masks must be
changed to reflect the network number for each subnet). You must assign
a different address to any device on the LAN that was using one of the
addresses in the range of addresses subnetted to the twinax workstations.
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