Common Network Service Issues | CompTIA Network+ N10-007 | 5.5

In this video you will learn about common network service issues as it relates to mismatched MTUs, incorrect subnet masks, incorrect default gateways, duplicate IP addresses, incorrect DNS configurations, duplicate MAC addresses, expired IP addresses, exhausted DHCP scopes, rogue DHCP servers, untrusted SSL certificates, incorrect times, blocked TCP/UDP ports, incorrect host-based firewall settings, incorrect ACL settings, and unresponsive services.

Mismatched MTU (Maximum Transmission Unit)

Router interfaces have a parameter called the maximum transmission unit that defines the largest packet size the interface will forward. For example, a 1500-byte packet could not be forwarded via a router interface with an MTU of 1470 bytes.  A router attempts to fragment a packet that is too big unless the packet has its “don’t fragment” (DF) bit set.  If a packet exceeds an interface’s MTU & has its DF bit set, the router drops the packet.  Normally, the router responds to the sender with an ICMP (Internet Control Message Protocol) message indicating why the packet was dropped.  However, if a router is configured to not respond to such a condition by sending an ICMP, the packet is dropped without the sender being notified.  Such a router is called a blackhole router.  You can use the traceroute utility to help locate a blackhole router.

Incorrect Subnet Mask

When one host attempts to communicate with another host on the same subnet, the sending host sends an ARP request in an attempt to determine the MAC address of the destination host, rather than forwarding traffic to the sending host’s default gateway.  Therefore, if a host has an incorrect subnet mask, it could incorrectly conclude that another host is on its local subnet, when in reality the other host is on a remote subnet.  As a result, the remote host is unreachable from the perspective of the sending host.

Incorrect Default Gateway

If a host has an incorrect default gateway configuration, traffic from that host is not forwarded off that host’s local subnet.

Duplicate IP Address

Hosts on a subnet should have unique IP addresses.  If two hosts are configured with the same IP address, unpredictable traffic patterns for those hosts can occur.

Incorrect DNS Configuration

Because hosts often use DNS to resolve domain names to IP addresses, if a host has an incorrect DNS configuration, that host will be unable to browse the Internet using domain names (as opposed to IP addresses).

Duplicate MAC Address

Although it’s rarer than a duplicate IP address on the network, you could experience a duplicate MAC address.  This is typically caused by a software configuration of MAC addresses.

Expired IP Address

A leased DHCP address that has expired for a workstation could cause issues as well.

Exhausted DHCP Scope

Although it’s rare, you may have a DHCP scope that has run out of addresses to lease; this could be the result of a DHCP exhaustion type of attack.

Rogue DHCP Server

This security concern could certainly cause troubleshooting headaches on the network due to the incorrect IP configurations that could result.  This is often the result of users bringing in their own devices, either maliciously or accidentally.

Untrusted SSL Certificate

If using certificates for authentication, you might have an issue with the SSL certificate that is preventing such authentication.

Incorrect Time

Because many network configurations rely on the correct time, you might have problems in the network due to a workstation, server, or network appliance with the wrong time set.

Blocked TCP/UDP Ports

Perhaps your trouble ticket is the result of blocked TCP or UDP ports.  This is often the result of a misconfigured security device in the network path.

Incorrect Host-Based Firewall Settings

Another security area that could cause problems is host-based firewalls.

Incorrect ACL Settings

Access control lists can have a direct effect on network traffic & could be the cause of one or many trouble tickets.

Unresponsive Service

A service that has failed in the network is another common source of problems.


  1. Sequeira, A. CompTIA Network+ Cert Guide.