IP Configuration Issues
- IP issues exist at the Network layer
- first rule out Physical and Data Link layer issues
- then check basic addressing and protocol configuration
- if host cannot perform neighbor discovery to contact any other hosts on the local network:
- first use
ipconfig or ip or ifconfig to verify host configuration
Incorrect IP Address
- each end system host must have:
- same netmask as its neighbors
- IP address that produces a valid, unique host address within that subnet
- neighbor is another host in the same layer 2 broadcast domain
- e.g., for subnet
192.168.1.0/24
- Host A:
192.168.1.10 Mask: 255.255.255.0
- Host B:
192.168.1.11 Mask: 255.255.255.0
- Host C:
192.168.0.21 Mask: 255.255.255.0
- Hosts A and B have valid configurations
- Host C is unable to communicate on the network
- non-default masks are more difficult to identify valid host ranges
- E.g., for subnet:
192.51.100.16/28
- Host A:
192.51.100.10 Mask: 255.255.255.240
- Host B:
192.51.100.11 Mask : 255.255.255.240
- Host C:
192.51.100.21 Mask: 255.255.255.240
- network prefix boundary is in last octet
- converting to binary will show that Host C is not in same subnet
Incorrect Subnet Mask
- if subnet mask is incorrect,
- can receive communications
- but misroutes replies
- thinks the other hosts are on a different subnet
- replies may go to default gateway rather than direct
- E.g., longer prefix
- Host A: IP:
192.168.1.10, Mask: 255.255.255.0
- Host B: IP:
192.168.1.11, Mask: 255.255.255.0
- Host C: IP:
192.168.1.21, Mask: 255.255.255.240
- Host C has longer prefix
- thinks it needs to route to different subnet to communicate to A and B
- causes packets to go to router
- E.g., 2 shorter prefix
- Host A: IP:
192.168.1.10, Mask: 255.255.255.0
- Host B: IP:
192.168.1.11, Mask: 255.255.255.0
- Host C: IP:
192.168.1.21, Mask: 255.255.0.0
- Host D: IP:
192.168.0.10, Mask: 255.255.255.0
- no problem for Hosts A, B, C if attached to switch
- C won’t be able to contact D, thinks D is on same network