Describe IP operations

Exam: Cisco 200-301 - Cisco Certified Network Associate (CCNA)

One important topic under exam number 300-101 route is network principles. Under this section there is a sub topic "describe IP operations". We will be discussing the same in details so that it can help the candidates to prepare better for the CCNP exam.

ICMP Unreachable and Redirects

The ICMP is generated by the host gateway and it indicates that the packet was discarded as the destination was not reachable. This will not be generated for a multicast traffic. The ICMP unreachable can be of fifteen types and these are:

  1. Network unreachable- it has a code value of 0. In this the datagram cannot be delivered to the network that is specified in the network ID portion of the IP address. This means that there was a problem with the routing and was caused by a bad address.
  2. The second is the host unreachable. This has a value code of 1. In this the datagram was delivered to the network that is specified but it could not be sent to the host who was mentioned in the address. This also implies a routing issue.
  3. The protocol unreachable is the next. The value code of this is 2. In this the destination port that was specified as UDP or TCP header was not valid.
  4. Fragmentation needed and DP set - the MTU is always smaller than the packet size. The router is not allowed to fragment the packet. This message is very often used in a very clever way. This is done by intentionally sending messages of increasing size to understand the maximum transmission size that the link can take. This process is also called the MTU path discovery.
  5. Source route failed - this is generated if the source route was specified for the datagram in the option but the router could not forward the datagram to the next step in the route.
  6. Destination network unknown - this is not used any more.
  7. Destination unknown host - in this the host that is specified is not known. This is generated by a router call made to the destination host. This usually means a bad address.
  8. Source host isolated - this is again not used anymore.
  9. Communication with the destination network is prohibited administratively -in this the source device will not be allowed to send the network where the destination device is located.
  10. Communication with the destination host is administratively prohibited - in this the source device is allowed to send network where the destination device is located but not to the particular device.
  11. Destination network is unreachable for the type of service - in this the network that is specified in the IP address cannot be reached due to the inability to provide the service that is clearly specified in the "type of service" field of the datagram header.
  12. Destination not reachable for the type of service - in this the destination host that is specified in the IP address cannot be reached due to the inability to provide the "type of service" mentioned in the datagram header.
  13. Communication administratively prohibited - in this the datagram cannot be forwarded as the filtering blocks the message based on the contents.
  14. Host precedence violation - this is sent by the first hop router when the precedence value in the "type of service field" is one that cannot be permitted.
  15. Precedence cutoff in effect - this is sent by the datagram that is in the receiving end. The precedence value should be lower than the minimum allowed for the network at that time.

ICMP redirect

This mechanism is used by the host to notify that a better next hop is available to exit from the network. The CISCO routers will send the ICMP redirect message when the following conditions are met:

  1. The subnet or the network of the source IP address is same as that of the subnet or network of the next hop IP address.
  2. The interface where the packet comes into the router are same as the interface on which the packets are routed out.
  3. The datagram is not source routed.
  4. The kernel is such that it is configured to send redirects.

IPv4 and IPv6 fragmentation

The IPv4 options is used to test networks and for debugging. The different types of IPv4 are:

  1. Record route -Each of the routers on route record’s the address in the header. This information is sent to the originator by the destination. It can be limited only to 9 hops as that is the maximum the header can hold.
  2. Source route -in this the sender specifies the route through the network. This format is used as the record route. Only the sender can populate the header of the IPs.
  3. Timestamp - it is similar to the record route but the only difference is that each router also adds to the time span.

The IPv6 extension headers are of the following types:

  1. Hop by hop EH - this is used to support the router alert option. It is an important part of MLD (multicast listener discovery).
  2. Destination EH-it is used in IPv6 mobility and also source routing.
  3. Routing EH - it is required for IPv6 mobility. Sometimes the routing EH must be disabled to protect the routers from DDoS.
  4. Fragmentation EH -this is used to support communication using the fragmented packets.
  5. Mobility EH - this is also used for mobile IPv6 service.
  6. Authentication EH -it is similar to the IPv4 authentication header.
  7. Encapsulation security payload EH -it is a similar format that is used in the IPv4 ESP header. The ESH is encrypted as a result it cannot be assessed by the intermediary network devices


TTL stands for time to live. It is a 8 bit field in a IP packet. The main function of this is to ensure that the traffic is prevented indefinitely. It is examined at switch level. It does nothing to help the layer 2 loops. The TTL is used by TCP, DDP and ICMP.

We hope that this chapter on "describe IP operations" will help you to understand the topic better. Do prepare on these lines and you will surely be able to do well in the exam.

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