ELSA Cable - Manual (Page 46)

Operating modes and functions
ELSA MicroLink Cable
38
FTP server. The router knows the IP address of the FTP server in the LAN from the entry
in the service table (in
ELSA LANconfig in the 'TCP/IP' configuration section on the
'Masq.' tab or in the Setup/IP Router Module/Masquerading/Service
Table menu). The packet is forwarded to this computer. All packets that come from
the FTP server in the local network (answers from the server) are hidden behind the IP
address of the router.
The only small difference is that:
࡯
Access to a service (port) in the user's LAN from outside must be defined in advance
by specifying a port number. The destination port is specified with the LAN IP
address of, for example, the FTP server, on a service table to achieve this.
࡯
When accessing the Internet from the LAN, on the other hand, the router itself
makes the entry in the port and IP address information table.
The table concerned can hold up to 2048 entries, that is it allows 2048
simultaneous transmissions between the masked and the unmasked network.
After a specified period of time, the router, however, assumes that the entry is no
longer required and deletes it automatically from the table.
Which protocols can be transmitted using IP masquerading?
Naturally, only those which also communicate using ports. Protocols working without
port numbers or using ports above IP in the OSI model cannot be masked without special
treatment.
The current version of router implements masquerading for the following protocols:
࡯
TCP (and all TCP-based protocols such as FTP, HTTP etc.)
࡯
UDP
࡯
ICMP
DNS forwarding
Names rather than IP addresses are generally used to access a server over the Internet.
Who knows which address is behind 'www.domain.com'? The DNS server, of course.
DNS stands for Domain Name Service refers to the assignment of domain names (such
as domain.com) to the corresponding IP addresses. This information must be constantly
updated and be accessible all over the world at any time. DNS servers holding long
tables containing IP addresses and domain names exist for this purpose.
If a computer calls up a home page from the intranet, it first sends out a DNS request:
“What is the IP address associated with www.domain.com?”
This request is dealt with as follows if the cable modem is registered as the DNS server
for the workstation computers: