Basically every device that is connected into a network and is considered manageable has an IP address or can be assigned an IP address.
Routers have IP addresses assigned to their interfaces because simply Routers need IP addresses to route packets. if you bought for example a Cisco router, open the box, take the router out, sit it onto your desk plug it into the network would it do the job it is supposed to do? can you imagine a router that doesn't know how to route:-)? well, it is a dump router at first until you tell it this your Ip addresses and this is how you can route. so really to be intelligent and able to do its function it requires IP addresses.
Switches not all should have IP addresses unless they are manageable. if you got a simple unmanageable switch where you can't monitor what is coming in and out of the switch or being able to shutdown some ports and open others then why we would want that switch to be assigned IP address?
switches in concepts are different than routers so we can have dumb switches that simply switch with less intelligent. but if you got a sophisticated switch that would allow you to look inside it to managed it, monitor it, then you would definitely need this switch to be able to assign an IP address. Please note in switches this IP address is assigned to what we call the management interface.
management interface! this might be a weird hearing at first but it is said that because you first use this to manage the switch and second because you really do not assign IP addresses to each and every interface or port on the switch, so because you are unable to do such assignment you will simply assign a virtual interface an IP address that can be access from which every interface the request might come in. of course this we are not taking into considerations VLANs because we are trying to keep it simple in here.
Hubs do not do intelligent switching or anything fancy, they are really dumb, they simply take what is in and forward it to every and each port on it. so they don't help in management or in monitoring your network. so we really don't want to assign them IP addresses because we really can't do anything to them even if we got allowed to do so. so a hub can't be assigned an IP address because there is no benefit of doing so.
I guess you heard about the old Bus Topology network where all devices are connected to the same cable. well hubs were introduced to shorter this Bus cable topology:-). they simply took this cable in the ceiling where all PCs are plugged into it in Bus structure and put it in a little box called a Hub.
what is a gateway? or what is the difference between a gateway and a router? well they are the same. but gateways used to refer to a box that would be a gate between two different networks. that it would router between two different networks like a TCP/P network and IPX/SPX network. But really nowadays all networks uses TCP/IP and for that the term router is used because it simple routes packet from one source to another or from one subnet to another.
if you take a 10 PCs plug them in a room or a floor using cables that would be consider a LAN right! and if one PC wants to talk to another it will broadcast or send frames down the cables to reach the desired PC. do we need a router or a gateway? well no, because we are living on the same network and if one PC wants to reach the others it would simple broadcast the data.
so in order to reach other networks we introduce routers or gateways into our networks. for not each host to be responsible on detecting other networks and communicating with other networks, we use routers instead.
routers a dedicated devices which really in its hardware design does not differ much from any PC you might know but we drop on this hardware a dedicated operating system that would eat, breath and think in routing. that would do nothing else except rotuing between networks and building a route table to help PCs on different networks reach each others.
for this when a PC at network 192.168.1.0 wants to reach another PC on network 172.16.1.0, well these two PCs are not on the same subnets right? not even on the same network address. so the PC will say "well, I don't know where 172.16.1.x locate" so I will send my data to my default router (gateway) and the gateway will handle that for me.
I hope that was helpful to you.
