ELSA Cable manual

Operating modes and functions 37

Cable TV net-	LAN of the user
work

Cable IP address LAN IP address

MCNS

ELSA MicroLink	Hub
Cable

The cable modem is therefore assigned a cable IP address and a LAN IP address, each with its own appropriate network mask. Use the 'Masquerade' option to inform the cable modem which of the two addresses to use when transferring the packets.

K'Off': No masquerading.

K'on': Use this entry to apply the cable IP address that was assigned during the registration at the headend by the network operator.

If the cable modem is used as an IP router without masquerading, make sure that IP/RIP is enabled. When using it as an IP router with masquerading, IP/RIP should be disabled.

How does IP masquerading work?

Masquerading makes use of a characteristic of TCP/IP data transmission, which is to use port numbers for destination and source as well as the source and destination addresses. When the router receives a data packet for transfer it now notes the IP address and the sender's port in an internal table. It then gives the packet its unique cable IP address and a random new port number. It also enters this new port on the table and forwards the packet with the new information.

The response to this new packet is now sent to the cable IP address of the cable modem with the new sender port number. The entry in the internal table allows the router to assign this response to the original sender again.

You can view these tables in detail in the router statistics (see also 'Status').

Simple and inverse masquerading

This masking operates in both directions: The local network behind the cable IP address of the router is masked if a computer from the LAN of the user sends a packet to the Internet (simple masquerading).

If, on the other hand, a computer sends a packet from the Internet to, for example, an FTP server on the LAN, from the point of view of this computer the router appears to be the

ELSA MicroLink Cable