Cabletron Systems CSX400 manual Subnet Mask, Local Segment, Target Segment, Hop Count Limit, 4-60

Bridging

Subnet Mask

A subnet mask is used by a device to determine whether a destination address exists within its own subnetwork (logical division of the network by router or gateway) and can be reached directly, or whether it is unknown and therefore must be delivered to a router (as speciÞed by the deviceÕs IP routing table or default gateway address).

A subnet mask should be set at the device if it will issue SNMP traps in a routed environment, so that the trap messages it generates will be routed correctly.

A subnet mask acts as a Þlter for destination IP addresses. It is a 32-bit quantity in which all bits that correspond to the network portion (both site and subnet identifying bits) of the deviceÕs IP address are set to 1, and all bits that correspond to the host portion are set to 0.

The device will logically AND a destination trap IP address with the subnet mask to determine which portion of the address identiÞes the network/subnetwork. The device then compares the result on a bit-to-bit basis with the network identifying bits in its own IP address. If the network portions match, the bridging device transmits the trap onto its subnetwork. If they do not match, the device transmits the trap through a router or gateway.

This Þeld can only be edited (with the correct security access) via Local Management for the device (or the MIBTools utility). Refer to the appropriate device-speciÞc UserÕs Guide for more information.

MAC Address

This Þeld displays the Media Access Control (MAC) layer address which identiÞes the ports/interfaces of the bridging device on a network. This six-byte address is set at the factory and is unique to each interface. Each byte is identiÞed in bit order starting with the most signiÞcant bit. You cannot conÞgure this Þeld.

The following Þelds apply to the Source Route ConÞguration window:

Local Segment

This Þeld displays the unique segment number that identiÞes the segment attached to the selected interface (either of the Token Ring or FDDI interfaces). The bridge adds the Local Segment number to the routing information Þeld of source route discovery frames. Valid values range from 0 to 4095.

Target Segment

This Þeld displays the unique segment number of the target segment that the source routed frame will be forwarded to. Valid values range from 0 to 4095.

Hop Count Limit

The maximum number of routing descriptors (i.e., bridge hops) allowed for an All Routes Explorer or a Spanning Tree Explorer frame received by the device. This will reduce the unnecessary propagation of explorer frames through the network.

4-60 Source Route Configuration