Subnetting; Installing The Ibm 5500 Control Unit Into An Existing Network - IBM 5500 Operational, Installation, And Maintenance Manual

Subnetting

Installing the IBM 5500 Control Unit into an Existing Network

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As part of planning the installation of your 5500 Control Unit, determine the
range of addresses to be used by each network port of the 5500 Control Unit.
The twinax port allows up to 56 twinax-attached workstations to exploit the
TCP/IP protocol for access to network resources.
The 5500 Control Unit is unique because it allows the use of subnetting of an
address space to define the addresses that can be used for the twinax
workstations without making subnet mask or default gateway changes to
existing LAN workstation or router configurations. This is commonly known
as subnetting. The 5500 Control Unit forwards IP packets allowing all devices
attached to the 5500 Control Unit to communicate with each other. Any TCP
request that is sent to an IP address in the network associated with any port of
the 5500 Control Unit will be forwarded to that port.
The 5500 Control Unit implements an extended ARP (Address Resolution
Protocol) agent to send packets to twinax-attached TCP/IP workstations based
on the twinax port using a subnet of the network assigned to one of the LAN
ports of the 5500 Control Unit. If the destination IP address is outside the
range of addresses known to the 5500 Control Unit (or any default gateways
configured for the workstation), then the request will fail.
If a LAN-attached device performs the function of a router to the Internet,
then the IP address of that device should be used as the default gateway for
that network segment. This action grants Internet access to all workstations
attached to the 5500 Control Unit provided that the address space used by
those workstations was properly coordinated so that your domain is known to
the InterNIC domain name server.
One of the most common applications of the IBM 5500 Control Unit is to
allow existing TCP/IP-capable twinax workstations that are remotely
connected to an AS/400 through a 5494 Control Unit to be properly migrated
to the IBM 5500 Control Unit. This application allows workstations to use
new e-business applications. Under this circumstance, or for any other
application where the IBM 5500 Control Unit is placed on an existing LAN,
you must decide whether to subnet the IBM 5500 Control Unit or to obtain an
additional set of addresses for the twinax workstations attached to the control
unit.
Should you decide to obtain a new address space, the routing tables in routers
that are upstream from the IBM 5500 Control Unit must be manually updated
to include the new address space. You will have to coordinate those addresses
with the InterNIC (if Internet access is desired) and default router addresses to
be configured in the twinax workstations. Manual table updates are required
because the IBM 5500 Control Unit does not run any routing protocols (for
example RIP and OSPF).
Planning, Installation, and Maintenance Guide
Chapter 2: Planning Your Installation