Hello,i am mahesh,i am a system admin and not so familiar with networks.My company has 4 branches connected thru routers(SHDSL) and each router has got its own default gateway.The internet is only in my head office and shared by branches thru router.The issue is recently i reconfigured my wireless modem after that i am not able to browse internet from branches except head office .I need help on this issue
You need to have the routers at each branch aware of each other's subnets via route tables if connected in a star design or just the central router aware of all of the networks if in a pin-wheel design. I'm assuming a star design for theoretical purposes.
eg(assuming netmask of 255.255.0.0)
Head Branch: 10.1.0.0
Branch 2: 10.2.0.0
Branch 3: 10.3.0.0
If you're at Branch 3 and you send a packet to 10.2.x.x, then it needs to get routed to Branch 2. If you're at Branch 3 and you send a packet to an unknown subnet, then that packet needs to get forwarded to the router at the Head Branch. That router should then know if the packet needs to head to the internet or to another internal network.
Typically, all routers should know all of routes so they can directly forward packets to their respective networks. If a packet has an unknown network, then it goes to the internet for further routing.