Communication Networks/IP Protocol and ICMP

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Internet Protocol[edit | edit source]

Internet Protocol (IP) is the Internet layer protocol that contains address information for routing packets in Network Layer of OSI model.

We've talked so far about TDM (Time-Division Multiplexing) techniques, and we've also talked about how different packets in a given network can contain address information, that will help the routers along the way move the data to its destination. This page will talk about one of the most important parts of the internet: the IP Protocol.

IP, as an integral part of TCP/IP, is for addressing and routing packets. It provides the mechanism to transport datagram across a large network. In more detail, the main purpose of IP is to handle all the functions related to routing and to provide a network interface to the upper-layer protocols, such as TCP from Transport Layer. Applications use this single protocol in the layer for anything that requires networking access.

What is IP?[edit | edit source]

The Internet Protocol is essentially what makes the Internet different from other digital networks (ARPANET, for instance). IP protocol assigns a unique address, called the "IP Address" to each computer in a network, and these IP addresses are used to route packets of information from a source to a destination. IP protocol calls for each device in the network to make the best effort possible to transmit the data, but IP doesn't guarantee that the data will arrive. If you are looking for a guarantee, you will have to implement a higher-level protocol (such as TCP).

From the OSI model, the IP Protocol is a Network-Layer Protocol.

The IP address is a different number from the "MAC Address" that is also found inside a computer. The IP address is a 32bit value that is unique among computers in a given local network. A MAC address is a larger number that is unique in the entire world. However, it is very difficult to route packets according to the MAC address.

IP also specifies the header that packets must have when traveling across the Internet. This header is called the IP header, and will be discussed in the next chapter.

The IP Protocol also specifies that each IP packet must have an error-checking code attached to the end of the packet. This error-checking code, called the "Cyclic Redundency Check" or CRC Checksum is capable of helping the receiving computer determine if the packet has had any bit errors during transmission. The CRC code is much more powerful at detecting errors than a single parity bit is, but CRC can be time consuming to calculate.

IP Address[edit | edit source]

IP address is a set of numbers identify any packet sends from sender to receiver on IP network in the Internet. It is a software address associated with interfaces, 32-bit information as a hierarchical address structures to handle a large number of addresses, assigns to each machine as interface that designs to communicate between hosts in different network. Dotted-decimal notation usually use as for easy understanding purpose.

An IP address consists of two parts, Network Address and Host Address. Network Address for identify each network, and Host Address for identify individual machine.


File:IP Address.jpg

One would wonder what the IP address information of the current machine that connecting to the network, we could use ipconfig command to find out:


Figure 6 ipconfig Command for Address Information

The current machine turns out to have the IP address of The next figure will show a physical interface representation corresponds to the IP address, and notice the first 16 bits are representing the network address, which will stay the same within its own network:

File:IP Connection.jpg

Figure 7 Network Connection

There are different classes of networks, based on the network size, as shown in the following:

File:IP Address Class.jpg

Figure 8 IP Address Class Identification

Notice that bits in the beginning of each class set are defines by address schemes, which will not be used. So, the network address ranges for each class are as follows:

File:Network Address Range.jpg

However, because of the address demanding and shortage, there IP addresses exist that reserved for special purposes and sets for private network.

File:Reserved IP.jpg

File:Private IP.jpg

IP Packet Format[edit | edit source]

IP Header[edit | edit source]

The IP header is a large field of information that is appended to the beginning of the packet. The IP header includes a large amount of information about the packet, including the source IP address, and the destination IP address. Also, the IP header (IPv6 and up) includes information about the local area networks for both the source and the destination terminals.

Each IP header contains information relates to data sends from upper layers for identifying the destination, and is shown as follows:

File:IP Packet.jpg

Figure 4: IP Packet

File:IP Packet Field.jpg

Here is a snapshot of an IP packet capture on Ethereal:

File:IP Packet Ethereal.jpg

Figure 5: IP packet captured from Ethereal

Notice in the protocol field that captured above, it indicates TCP. Since the header does not have any protocol information for the next layer, it simply directs IP to pass the segment to TCP at the Transport Layer. All other fields correspond to the description above.

CRC Checksum[edit | edit source]

The CRC checksum is a 16bit data item appended to the end of an internet IP packet. The CRC contains a number that the receiver runs through a particular algorithm, and can then determine if the packet is correct, or if there is an error.

Modulo-2 Arithmetic[edit | edit source]

This section will be a short primer on Modulo-2 Arithmetic

Calculating the CRC[edit | edit source]

Using the CRC to Find Errors[edit | edit source]

IPv4 and IPv6[edit | edit source]

Internet Control Message Protocol (ICMP)[edit | edit source]

Internet Control Message Protocol is used to pass information related to network operation between hosts, routers and gateways in network level.

There are four major functions as follows:

  • Announce network errors when the network being unreachable.
  • Announce network congestion when a router over-buffer due to too many packets transmitting.
  • Assist Troubleshooting when packets send over a network to compute the loss percentages and round-trip times.
  • Announce Timeouts when TTL of an IP packet drops to zero, where a packet being discards.

Please note that ICMP packets are crafted at the IP layer and thus does not guarantee delivery.

ICMP, the Internet Control Message Protocol is a counterpart to the IP Protocol that allows for control messages to be sent across the network.

Classful Address Scheme[edit | edit source]

Subnetwork[edit | edit source]

Subnetwork (Subnet) is used to group computers in the same network that has IP address with the same network address. Subnet is one of the solutions for resolving the shortage of addresses and to help utilizing the address assignment in the network. Subnet mask is introduced to have the network breaks into subnetworks in order to provide a hierarchical routing architecture.



Subnet mask:

Slash notation is introduced to identify the number of bits turn on. When the Internet Service Provider (ISP) allocates addresses to the users, these addresses will be in a slash notation form:



In addition to the advantage of grouping computer, there are some benefits from subnetwork:

  • Reduced network traffic
  • Increase network performance
  • Simplified management

Classless Interdomain Routing (CIDR), also known as supernetting, is another solution for shortage of addresses. The basic idea is the same as subnet. The only different is that host address is occupying bits from the network address, which help for address wasteful avoidant purpose.



Subnet Masks[edit | edit source]

Classless Interdomain Routing (CIDR)[edit | edit source]

Summary[edit | edit source]

Internet Protocol (IP) is responsible for addressing and routing packets in the Network Layer (layer 3) of the 7 layer OSI model. Messages are transmitted hop by hop in this layer, and each node's interface has a unique IP address for identification in the network. It allows datagram to transport across a large network.

Internet Control Message Protocol (ICMP) is a way to send error messages or perform network diagnostics across a network. Two of the most common tools utilizing ICMP are Traceroute and Ping.

Exercises[edit | edit source]


  1. What is the Class C private IP address space?
  2. What is the subnetwork number of a host with an IP address of
  3. What is the subnetwork number of a host with an IP address of
  4. The network address of provides how many subnets and hosts?
  5. You have a Class B network ID and need about 450 IP addresses per subnet. What is the best mask for this network?
  6. You router has the following IP address on Ethernet: How many hosts can be accommodated on the Ethernet segment?
  7. If a company calls for technical support regarding to its malfunction network, what are the four basic steps to perform a IP addressing troubleshooting?
  8. If an Ethernet port on a router were assigned an IP address of, what would be the valid subnet address of this host?
  9. (T/F) ICMP messages are encapsulated in IP datagrams.
  10. (T/F) Ping program uses “TTL” field to detect if a destination host is alive.


  1. –
  4. 8 subnets, 8190 hosts each
  6. 4094
  7. (a) Ping (b) Ping local host IP address. (c) Ping default gateway. (d) Ping the remote server.
  9. True
  10. False - Ping waits for an "echo response" packet from the target.

Further reading[edit | edit source]