Almost all consumer Internet access links use only a single IP address and many business links as well. A single IP address can't be shared directly but extremely commonly (source) NAT is used.

A modem per se can't be shared but when a connection is dialed up by a router and then NATted, the connection can be shared just with xDSL, fiber or anything else. From the network perspective*, the modem is just the line interface like a fiber transceiver or a cable modem, not much else. It's up the device using it what you can do with it.

Actually using single-line analog and ISDN dial-up connections to share to a whole network wasn't uncommon until the late 1990s. Hard to imagine nowadays...

*"from the network perspective" = from the packet network perspective - A modem provides a serial link to an overlay network on top of the telephone network. The PPP protocol on top of the serial link converts the serial line to a multiplexed 1:N (or N:N) packet network interface and running IP on top of that eventually connects to the Internet.

Almost all consumer Internet access links use only a single IP address and many business links as well. A single IP address can't be shared directly but extremely commonly (source) NAT is used.

A modem per se can't be shared but when a connection is dialed up by a router and then NATted, the connection can be shared just like xDSL, fiber or anything else. From the network perspective*, the modem is just the line interface like a fiber transceiver or a cable modem, not much else. It's up the device using it what you can do with it.

Actually using single-line analog and ISDN dial-up connections to share to a whole network wasn't uncommon until the late 1990s. Hard to imagine nowadays...

*"from the network perspective" = from the packet network perspective - A modem provides a serial link to an overlay network on top of the telephone network. The PPP protocol on top of the serial link converts the serial line to a multiplexed 1:N (or N:N) packet network interface and running IP on top of that eventually connects to the Internet.

Almost all consumer Internet access links use only a single IP address and many business links as well. A single IP address can't be shared directly but extremely commonly (source) NAT is used.

A modem per se can't be shared but when a connection is dialed up by a router and then NATted, the connection can be shared just with xDSL, fiber or anything else. From the network perspective*, the modem is just the line interface like a fiber transceiver or a cable modem, not much else. It's up the device using it what you can do with it.

Actually using single-line analog and ISDN dial-up connections to share to a whole network wasn't uncommon until the late 1990s. Hard to imagine nowadays...

*("from the network perspective" = from the packet network perspective - a modem provides a link to an overlay network on top of the telephone network that eventually connects to the Internet)

Almost all consumer Internet access links use only a single IP address and many business links as well. A single IP address can't be shared directly but extremely commonly (source) NAT is used.

A modem per se can't be shared but when a connection is dialed up by a router and then NATted, the connection can be shared just with xDSL, fiber or anything else. From the network perspective*, the modem is just the line interface like a fiber transceiver or a cable modem, not much else. It's up the device using it what you can do with it.

Actually using single-line analog and ISDN dial-up connections to share to a whole network wasn't uncommon until the late 1990s. Hard to imagine nowadays...

*"from the network perspective" = from the packet network perspective - A modem provides a serial link to an overlay network on top of the telephone network. The PPP protocol on top of the serial link converts the serial line to a multiplexed 1:N (or N:N) packet network interface and running IP on top of that eventually connects to the Internet.