WHY 2 PC is not getting an IP address from DHCP Server ?
On Figure 1.1 , We can see a Server is connected to Router, other hands it is connected with a switch and 3 end devices. DHCP – Auto assign IP address that is very handy. DHCP Server is setup in Server-PT Server0, But those 3 computers can’t get a IP address. If computers can’t get an IP address, they can’t reach to the file share server or other resources.
- Switch interface Fa0/2 – PC0
- Switch interface Fa0/3 – PC0 (1)
- Switch interface Fa0/4 – PC0 (1)(1)
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DORA PROCESS
Once your computer is connected by wired cable or wireless, and enable DHCP. Your computer will send out DHCP broadcast known as DORA.
Pictures below you can see how is the process.
WHY PC CAN’T GET IP ADDRESS
Back to why computer can’t get a As we can see figure 1.2, Packet reach to Router. But the router didn’t forward DHCP packet to the Server. Basically, Router will drop broadcast message. Router will drop DHCP message from computers sent.
We need to use a command on the Router. “ip Helper”
On figure 1.3 , when I enable IP helper, then computer will obtain IP address with no problem.
AIPA ADDRESS
Another issues came, why the other 2 PC is getting an weird IP address “169.254…” but not ” 192.168.1.0″. When a computer can’t find DHCP address, they will have AIPA address “169.254.0.0” instead. By default for a Switches, all port is belong to VLAN 1, native VLAN too.
On a image at the beginning (” Switches VLAN Brief “), I specifically set Port Fa0/3 & Fa0/4 belong to VLAN 10. VLAN 10 is a different network, That’s why those 2 PCs can’t get IP address from DHCP Address.
