1. Introduction to Networking
IP Configuration Troubleshooting

2. Network Connection Details
When the network isn’t working we may need to collect data in order to troubleshoot the network and see what is happening and why it isn’t working
In Windows, to do this we can click on the connection in the Network and Sharing Center, then click on details to get our information
We can also use IPconfig from the command prompt

3. IPconfig
IPconfig will give us our basic internet properties including IP address, subnet mask and default gateway. This is useful to diagnose basic problems, but we may need more info. In that case, we can use IPconfig /all

4. IPconfig /release and /renew
IPconfig /release forces our NIC to give up its IP address. This can be useful if our IP address is misconfigured and we need to give up the old data in order to be assigned new
IPconfig /renew forces the NIC to send out a request to the DHCP server for an IP address
If we do /release and /renew when everything is working right, our IP address usually won’t change since the lease won’t be up yet, but when something is wrong we can be given a new proper IP address and information

5. Example of APIPA address
If we have a computer set up to use DHCP, but it is having problems accessing the internet, or it is not connected to parts of its subnet, it may have been assigned an IP address from APIPA
Signs APIPA gave us our IP address
No default gateway
Network address starting with
Subnet mask of

6. Setting Up Alternate Configuration
To set up how our interface gets its IP address when the DHCP server is down, click on the connection in network and sharing center. Go to properties, then IPv4, properties, alternate configuration
We can let it automatically assign an IP address or we can have a static IP assigned in the case of DHCP outage
The benefit of a static alternate IP address is that we can assign a default gateway and DNS, allowing us access to the internet

7. IFconfig Linux version of IPconfig
Need to be root to change, any user can view it
To change the IP address and subnet mask for example, you would write ipconfig ens32 (NEW IP) netmask (NEW MASK)

8. ENS32 (or eth0) and LO
ENS32 (Or eth0 if you’re using an older version of Linux) will be the wired or wireless interface
LO will be the loop-back adapter to the local host. This will exist even if you don’t have an ethernet interface, since it is used by various services running on the system to send info to each other

9. Anatomy of IFconfig HWaddr - Hardware address, or Mac address
Inet addr - Internet address, or IP address
Bcast - Broadcast IP for our subnet
Mask - Subnet mask for our subnet
RX Packets - number of packets received
TX Packets - number of packets sent

10. Recognizing Different States
Static IP - DHCP Enabled - no, No DCHP server line
DHCP configuration - DHCP Enabled - yes, IP address on DCHP server line
Rogue DHCP - DHCP Enabled - yes, wrong DCHP IP address. Possibly multiple hosts with incorrect information with the same DHCP address
APIPA - DHCP Enabled -yes, No DHCP server line, IP address between and and subnet mask of
Alternate configuration DHCP Enabled -yes, No DHCP server line, IP address other than APIPA

11. Netstat Netstat stands for network statistics
Netstat without modifiers will show me a list of all active incoming and outgoing connections, the port they use, the IP address they are connected to and their status
Netstat -a will also include any listening ports
Netstat -es
Netstat -r

12. Netstat -es
Netstat -es gives the ethernet network statistics. This includes packets sent and received, packets discarded and errors caught and current connections

13. Netstat -r Netstat -r gives us the routing table for our computer
A netmask (another name for subnet mask) of will show us our default gateway, and a gateway of “On-Link” shows us what we are locally connected to

14. Nslookup and Dig Nslookup and dig both allow us to test our DNS server
We can enter nslookup with no options to use our default DNS server, or we can use nslookup -[SERVER IP ADDRESS] to use a different server
After entering nslookup we can enter website names to have the DNS server process. To quit, we type “exit”
When we get an answer, it will specify authoritative if our server is authoritative for the website we lookup, and non-authoritative if the server needs to lookup the site from a root server
Dig is available by default on Linux, but not Windows. It has similar uses

15. Dig