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Concepts TCP/IP

2-5

LAT multicast messages contain a rating for each service offered. Ratings range from 0 to 255; 0 means

the service is unavailable, while 255 means the service is available and has no current users. Ratings for a

given service may change over time. For example, the rating for a computer accepting logins will generally

change as its workload changes. Conversely, ratings for a modem are typically either 0 (in use) or 255 (not

in use). In the example above, the server with eight modems attached will continue to advertise that the

service “modem” is available (a 255 rating) until all eight modems are in use (a 0 rating).

Service ratings may concern even casual users, since they are used to determine which service a user will

be connected to whenever there is a choice. For example, if a user types Connect Hub and five nodes

offer service hub, the user will be connected to the least busy node automatically. In the case mentioned

above, where both the local Server and a remote node offered the same service, the ratings determine which

node will service the connection attempt.

Another major network management feature of LAT is the concept of service groups. Each port on the

Server and each service on the network can be thought of as belonging to one or more groups. When a user

or device requests a service connection, the LAT host will check to see if the groups to which the requester

belongs match those of the requested service. If any group number is common to both the requester and the

service, the connection attempt continues. If there are no common group numbers, the connection attempt

fails.

Note: There may be additional access restrictions on the service, such as password

protection.

Suppose Bob is logged into port 4 on his Server and the server manager has given port 4 access to groups

1, 7, 13, and 105. Bob, or anyone else using port 4, can only access services that have one of those group

numbers. Suppose Bob wishes to access a modem on a different server. If the server to which the modem is

attached allows access to groups 8, 12, 16, 42, and 105; Bob will be allowed to use the modem because he

and the modem service have group 105 in common.

Note: See Set/Define Port Authorized Groups on page 12-HIDDEN and Set/Define

Server Service Groups on page 12-49 for more information.

Group numbers also are useful to nodes because each node only needs to pay attention to multicasts that

involve its users’ groups. As a result, groups can hide services that would otherwise be visible. The server

manager can also hide services from a set of ports. Setting up and managing services and groups is discussed

in more detail in the Server Configuration chapter, next.

2.6 TCP/IP

2.6.1 IP Addresses

Every TCP/IP node on a network has an IP address, which is unique to that network and an Ethernet

Address, which is unique across all hardware in the world. The IP address provides information needed to

forward packets across multiple networks, if necessary.

The address is of the form n.n.n.n, where each n is a number from zero to 254, as in 192.0.0.1. The exception

is that there cannot be a zero in the last segment of the address.

Note: The number 255 is strictly reserved for broadcast packets.

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Lantronix MPS100 Guida di Installazione Pagina 19