Do I have to change ServerName in client.conf every time?
Yes, as the client-PC sends its printer commands with "ServerName 192.168.0.100" to the printer.
First of all, I am not a network expert.

This is how I solved the problem on my system.
As I have a
Sitecom WL-535 Wireless Network Broadband Router 54g the numbers are
ONLY valid when you use this router.
With another router they are probably different.
In between {} is the Dutch text.
From my Sitecom WL-535 router:
TCP/IP Configuration
Subnet Mask 255.255.255.0
Default Gateway 192.168.0.1
LAN
IP Address: 192.168.0.1
Subnet Mask: 255.255.255.0
Default Gateway: 0.0.0.0
DHCP Client Range: 192.168.0.100 - 192.168.0.200
(in the LAN section I can change the values)
I changed the network connection of the server-PC to which the printer is connected from
Automatic Query (DHCP) {Automatische aanvraag (DHCP)} to
Manual {Handmatig}
Adddress: {Adres:}192.168.0.201
Net mask: {Netmasker:} 255.255.255.0 (pull-down menu)
Gateway: {Poort:} 192.168.0.1
and /etc/cups/client.conf on the client-PC
ServerName localhost
ServerName 192.168.0.201
I hope this will help to solve the problem !

(it will make the wiki more complicated)