As Ron pointed out, the command will take on the other device.
Serial Cables can operate a variable clock rates, or frequencies. This is the duration a voltage is applied to the wire to transmit a single bit. Unlike Ethernet, who's frequencies are built into the specification itself (10BASE-T, 100BASE-TX, etc), Serial Links's frequency can be set differently on each link.
But crucial to transmitting data is for both sides of the cable to use the same Frequency. So a Serial Link elects one side of the cable to be the Clock Master (if you will), and the other side of the cable simply agrees to the frequency set by the Clock Master.
A Serial Cable has a DCE and a DTE side. The DCE side is elected as the Clock Master, so the device which has the DCE side of the cable connected to it must be the one which you enter the clock rate command.
To determine what side of the cable is connected, you can use the command show controllers s0/0/0. Four or five lines from the top you'll see it labeled as DTE or DCE.
You'll find that the ethernet sockets you're speaking of are actually a switch, on those smaller routers.
Cisco does the same thing with SOHO-type equipment, eg 867VAE, for the same reason.
[EDIT rewrote from here down]
With a 2811 you can add a switch module in just the same way, such as HWIC-4ESW. Switch ethernet ports are also called Layer 2 ports, and they are associated to a VLAN (by default VLAN 1) and can be set to be trunk ports, access ports and so on. These are expected to be faster for switch-like behaviour.
You can also add more "layer 3" interfaces, such as with HWIC-2FE. These have IP addresses, access lists and so on. These are expected to be faster for routing behaviour.
Somewhat confusingly, both types of interface are written the same way,but if you, for example, put an IP address on an L2 interface you get an error:
gw(config)#int fastEthernet 0
gw(config-if)#ip address 1.2.3.4 255.255.255.0
% IP addresses may not be configured on L2 links.
If you have a need for say, four interfaces with 192.168.0.1/24 through 192.168.3.1/24 on them, with routing and access control lists, NAT, etc, then there are a couple of common approaches.
Use lots of L3 interfaces in the obvious way
interface fastethernet 0/1
ip address 192.168.0.1 255.255.255.0
interface fastethernet 1/1
ip address 192.168.1.1 255.255.255.0
...
Use switch ports, with lots of VLANs [EDIT: corrected this to a config which is correct, from an 867VAE with 15.2]
interface FastEthernet0
switchport access vlan 100
no ip address
interface FastEthernet1
switchport access vlan 101
no ip address
...
interface Vlan100
ip address 192.168.0.1 255.255.255.0
interface Vlan101
ip address 192.168.1.1 255.255.255.0
...
Quite how you make VLANs varies on different routers. On a 2811 with the built-in interfaces (tested on 15.1) you can use an external switch with VLANS, and use "subinterfaces" which look like this
interface FastEthernet0/0
no ip address
!
interface FastEthernet0/0.100
encapsulation dot1Q 100
ip address 192.168.0.1 255.255.255.0
!
interface FastEthernet0/0.101
encapsulation dot1Q 101
ip address 192.168.1.1 255.255.255.0
...
Choosing between these approaches depends on the hardware you've got. There are sometimes performance complexities or subtle restrictions eg max ether HWICs for a chassis. My understanding is that some Cisco L3-capable interfaces can be put into L2 mode, and they enter this by the switchport command; but some require "subinterfaces", see below.
A structure which is extremely common is sometimes called "router on a stick". A router is configured with one L3 towards internet, and one set as a trunk port towards a switch. The switch has many VLANs; the router has many interface vlan lines.
Note also the so-called "Layer 3 Switch", which is a switch with a router in it that can be enabled in configuration. The difference between a router with a switch in it and a switch with a router in it is mostly one of engineering emphasis; for many purposes it's just a question of which has got the interfaces you want, and perhaps the routing protocols and higher level functions.
Best Answer
Routers route between networks, so each router interface needs to be in a different network. Don't be confused by what you think is a home router. The home router has separate networks on the router interfaces, but you are probably meaning a switch module in a home router. You need a switch or a switch module in a router for that, but those are not router interfaces.
For a router, there really is no real distinction between WAN and LAN. Some interfaces, e.g. serial interfaces, are more likely to be used for WAN connections, while some router interfaces, e.g. ethernet, are more likely to be used for LAN connections, but you could use just about any type of router interface for either connection type. There are no commands that say, "This is a WAN interface." You can connect two routers together with like interfaces.