There are some tool we could use to diagnose our system when it is unable to get an IP address.
Contacting the dhcp server:

We could start by contacting the DHCP server to request an IP address from the pool of 
addresses the DHCP has. 
We release the lease of the DHCP server using the below:

To get a new IP address, we use:

Checking the interfaces on the system:

We can check the network interfaces (Ethernet interface) on our system using the below command:

We can check the wireless interfaces using:

The "nmcli" tool:

It is a network manager tool, we might use to check the state of the network interfaces:

The mii-tool and network interfaces status:

We could use the "mii-tool" to get the status of the network interfaces as below:

The "ip addr" command:

We can also use the "ip addr" command which returns all the interfaces, their IP addresses, their status (UP/DOWN),...:

Activating an interface:

We could use the below command for enabling a network interface if it is disabled (status : DOWN):

Restating the network:

We could restart the network using the below command:

Assigning a static IP address:

We could add assign a static address to an interface in the "/etc/network/interfaces" as follows:

After saving the file, we should restart the network as mentioned above "systemctl restart network".
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