Interesting... Learn something new everyday!

A hub doesn't have to have a MAC address, the el cheapo hubs you can get are nothing more than a multiport repeater and mindlessly pass ethernet frames coming in any port back out all ports. Some hubs, like the 3Com ones bdogg mentioned, actually have configurable options that may include port segragation but basically any "managed" hub that needs in-band configuration will have a MAC address so it can get a corresponding IP address and be reachable over the wire. Even the smart hubs don't use their MAC address for anything other than a way to reach the hub's configuration, a layer 2 (MAC) or layer 3 (IP) identity is not required to perform the functions of a hub.

It becomes something else when it can filter based on layer 2 (MAC) addresses, it becomes a switch. It also technically qualifies as a switch if it can send data from one layer 2 standard to another, such as from 10MB to 100MB although the early 10/100 "hubs" didn't switch on every port but treated all the 10MB connections like a hub and then bridged that to all the 100MB connections.

Now, to address what you have true_atlantis. You have a cable "modem" and presumably the ISP (cable company) gives you one IP address based on one MAC address. You won't get more than one "public" IP address from the cable company unless this is something you paid extra for (or they are totally clueless.) You have a hub, if it is just called a "hub" and is something targeted for home/small networks it probably has no MAC address, no IP address and no configurable options. You have a Linksys wireless "router" that has an uplink port and can connect multiple wireless nodes. The question is does it also have extra ethernet ports (heck, just post the model number and someone here will probably know what all it is capable of.)

The way I would set this up, assuming the linksys has at least one ethernet port besides the uplink port, would be to plug the cable "modem" into the linksys' uplink port and the
hub into one of the linksys' other ethernet port(s). This wouldn't give you two networks, but you don't need two networks. With the linksys set at defaults, it would get a "public" IP from the cable and share that using NAT+private IP space with anything wired or wireless that is plugged into it (or plugged into any hub that is plugged into it.)

To have two networks, you need two routers. Or a router smart enough and with enough ports (say a Linux box with 3+ NICs, this is after all a Linux forum) to create the two networks (plus the ethernet connection to the cable modem, this network we call The Internet.)

So I guess the questions are what model is your linksys and does it have more ethernet ports and what is it that makes you belive you need two seperate networks?

Quick rundown:

Hubs: refers to "passive hubs" in colloquial use. All ports are promiscuous, half-duplex, and typically when auto-sensing speed of individual ports, breaks out 10mbps and 100mbps into separate segments. If all ports are the same speed, all connections are on the same segment.

Switching hubs: usually referred to as switches. Handle bonding of 10mbps and 100mbps (and 1000mbps) ports into one segment. Ports are not promiscuous ( Each port is on its own segment). Generally does not have a MAC address.

Managed hub: a switching hub, but where segments may be defined by the network administrator. Has a MAC address and generally an IP address (or may be managed a terminal connected to the RS232 or RS422/RS485 port). Ports may be configured to be promiscuous or on individual segments (see first point in this paragraph). Can act as a simple router to split network into VLANs, but generally the filtering is all-or-nothing and not NAT or PAT filtered.

Router: a managed switch with complex filtering capability, including PAT and NAT.