Why should client PCs always know the default gateway?

Question

Considering that my home computer has only one connection and it is to my ISP,
why does it need the default gateway to be configured (being DHCP client)?
Where else my connection can be deviated from this unique connection cable?
Is it possible to configure a router to which my PC is connected and, then,
the client PCs would not need to be configured with default gateway?
Or client PCs always have to know default gateway? 
Why?

Answer 1

How IP Packets are Routed on a Local Area Network
http://www.anitkb.com/2010/06/how-ip-packets-are-routed-on-local-area.html
Thanks, this helps but still confuses.
The article tells:
" Now that WK1 has WK2's MAC, it can send the packet directly to WK2"
A)
I do not understand what does it mean if computer has one outgoing network cable to switch.
It is switch who has different connections and, thereafter, can commutate different communication circuits.
So, I can only understand that WK1 somehow tells to switch to connect him directly to WK2 but not to router.
How?
B)
Were  WK1 and WK2 connected directly to a hub, instead of switcher, would they be able to communicate directly?
(The definition of a hub is that it takes a signal and broadcast it through all connections)
C)
Also, reading about switch-router-hub, I cannot understand which role is played by a client computer in communication
(of a hub, of a switcher?)?  Neither of them? What? 
A. I believe a fundamental understanding of what a switch and the differences and the similarities between a hubs, bridges and switches, as well as the OSI model, may be helpful at this time.
First, the OSI model is an industry standard defining how hosts communicate with each other. There are 7 layers. THe bottom layer has no intelligence and is the physical connection. As you go up the ladder, the intelligence increases.
7 Application (Application Gateways, Proxies, etc, operate at this level)
6 Presentation
5 Session
4 Transport (TCP, UDP, SPX live here and operate at this layer, NAT overlaps 3 & 4)
3 Network (IP & IPX live here. NetBEUI and DLC overlap 3 & 4. Routers operate at this layer)
2 Datalink (MAC addresses live here. Bridges and Switches operate at this level)
1 Physical (Hubs operate at this level)
Hubs, Bridges and switches allow ethernet hosts to communicate with each other, no matter what protocol is being used, whether TCP/IP, IPX/SPX, NetBEUI, DLC, etc.  They transmit packets on the network.
A hub is a Layer 1 device. It is a dumb device blatantly allowing all hosts to communicate to each other with no discerning source or destination addresses, whether MAC, IP, IPX or any other factor, in the packets.
A bridge is a Layer 2 device. It bascially has two interfaces connecting two network segments together. If a host on one side of the bridge, we'll call that SegmentA, is trying to communicate to another host on the same SegmentA, the
bridge will not allow the traffic to go to the other segment on its other interface, SegmentB. This helps reduce unnecessary traffic and reducing collisions, which slow down the network. If the host on SegmentA is communicating with a host on SegmentB, the
bridge allows the traffic. This is because a bridge has enough intelligence to read the Link layer, which has the MAC address (the physical address) of the interface or network card. It can read the source and destination MAC and determines whether to allow
that traffic across or not depending on where the source MAC is and where the destination MAC is.
Switches are basically multi-port bridges. When a switch intializes, it reads the MAC addresses of all connected devices and creates a "destination table." Notice I didn't say "routing table" since that is associated with IP addresses.
Therefore, if a host on port# 14 on a switch needs to communicate with a host on # 33 on the switch, the switch reads the source and destination MAC address in the Datalink Layer (Layer 2) and knows the destination is on port# 33 based on the destination
lookup table it created of all connected devices. It will then only send this traffic between the two ports. This essentially reduces unnecessary traffic on other ports increasing efficiency.
There are also Layer 3 Switches. They are combination switches and routers that can be managed where you can configure each port to either be switched or routed.
So to answer your questions:
A)
I do not understand what does it mean if computer has one outgoing network cable to switch.
It is switch who has different connections and, thereafter, can commutate different communication circuits.
So, I can only understand that WK1 somehow tells to switch to connect him
directly to WK2 but not to router.
How?
As explained, the switch simply discerns traffic by MAC address. The client side TCP/IP subsystem using the ANDING process, as I've explained earlier, and JM's blog explains, determines where the computer is sending the packets. A computer does not "Tell"
the switch or hub anything. It simply dumps the packet on the wire and the switch reacts to what it finds in the Datalink layer, and if a hub, it simply sends the traffic on all ports.
B)
Were  WK1 and WK2 connected directly to a hub, instead of switcher, would they be able to communicate directly?
(The definition of a hub is that it takes a signal and broadcast it through all connections)
It's not called a 'switcher.' It's called a 'switch.' As explained, a hub blatantly broadcasts traffic on all ports. It is up to the sending host and receiving host to read all packets and figure out what belongs to it or not. If a destination address doesn't
apply to a computer that hears the data, it simply ignores it.
C)
Also, reading about switch-router-hub, I cannot understand which role is played by a client computer in communication
(of a hub, of a switcher?)?  Neither of them? What? 
The computer is simply plugged into these devices. The devices have their job to do, and the computer has its own.
I hope that explains this part of your networking questioning.
Ace
Ace Fekay, MVP, MCT, MCITP EA, MCTS Windows 2008 & Exchange 2007, MCSE & MCSA 2003/2000, MCSA Messaging 2003, Microsoft Certified Trainer, Microsoft MVP - Directory Services. This posting is provided AS-IS with no warranties or guarantees and confers no
rights.