System information

140
AS/400 IBM Network Station: Techniques for Deployment in a WAN
5.5 Transparent Subnet Masking
In this scenario, we allow the twinax subnet access to the local LAN and
beyond. With the introduction of
transparent subnet masking , introduced in
V4R2 of OS/400, an addressing scheme for the twinax subnet can be
configured on the AS/400. This implementation allows IP over twinax devices
to appear as though they were on the local network.
Transparent subnet masking uses different masks over the same network ID.
This masking segments contiguous ranges of IP addresses together to use
for either twinax subnets or for remote LANs attached to the AS/400 system.
The transparency part comes into play when
Proxy ARP is enabled, which
happens automatically when the hosts on the network share the same
network ID. In effect, the subnetting within your network is transparent
because a router or gateway is
not
required to join the subnets.
Detailed information concerning
Proxy ARP is beyond the scope of this
redbook. Further information is found in the following sources:
•
AS/400 TCP/IP DNS and DHCP, SG24-5147
• RFC 826 which discusses Address Resolution Protocol
• RFC 1027 which discusses Proxy ARP
The twinax subnet requires a contiguous range of TCP/IP addresses
assigned to it. You
cannot
use any address at random from the pool and
dynamically allocate an address to a device on the twinax subnet. The
recommendation is that the maximum amount of TCP/IP addresses, which is
64, be assigned to the twinax subnet, if possible. This limit of 64 is actually
imposed by the workstations controller. It is not a true limit because the
workstation controller can only support up to 56 operational twinax devices.
By initially choosing to use the maximum number of TCP/IP addresses, there
is flexibility later to add devices to the subnet without having to change the IP
addressing scheme within the network. There are, however, smaller subnets
that can be configured for your twinax IBM Network Stations. Detailed tables
are included in
Appendix B of the IBM Network Station Manager Installation
and Use,
SC41-0664 . These tables outline exact ranges of IP Addresses
used in each scenario, depending on the quantity of Network Stations
needing support within the subnet.
Figure 48 on page 141 shows a simple example of a network that is using
transparent subnet masking and
Proxy ARP. All the networks and hosts are
on the same TCP/IP network ID, 10.1.x.x.