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After reading this section you should be able to

  • Understand methods of network design unique to TCP/IP networks, including subnetting, CIDR, and address translation
  • Explain the differences between public and private TCP/IP networks
  • Describe protocols used between mail clients and mail servers, including SMTP, POP3, and IMAP4
  • Employ multiple TCP/IP utilities for network discovery and troubleshooting

Designing TCP/IP-Based Networks

The following sections explain how network and host information in an IPv4 address can be manipulated to subdivide networks into smaller segments.


Subnetting separates a network into multiple logically defined segments, or subnets. Networks are commonly subnetted according to geographic locations, departmental boundaries, or technology types.

A network administrator might separate traffic to accomplish the following…

  • Enhance security
  • Improve performance
  • Simplify troubleshooting

The challenges of Classful Addressing in IPv4 (No subnetting)

The simplest type of IPv4 is known as classful addressing (which was the Class A, Class B & Class C network addresses).

Classful addressing has the following limitations.

  • Restriction in the number of usable IPv4 addresses (class C would be limited to 254 addresses)
  • Difficult to separate traffic from various parts of a network

Because of the above reasons, subnetting was introduced.

IPv4 Subnet Masks

Subnetting depends on the use of subnet masks to identify how a network is subdivided. A subnet mask indicates where network information is located in an IPv4 address.

The 1 in a subnet mask indicates that corresponding bits in the IPv4 address contain network information (likewise 0 indicates the opposite)

Each network class is associated with a default subnet mask…

  • Class A =
  • Class B =
  • Class C =

An example of calculating  the network ID for a particular device with a subnet mask is shown below..

  • IP Address =
  • Subnet Mask =
  • Resultant Network ID =

IPv4 Subnetting Techniques

Subnetting breaks the rules of classful IPv4 addressing.

Read page 490 for a detailed explanation

Calculating IPv4 Subnets

Read page 491 – 494 for an explanation

Important… Subnetting only applies to the devices internal to your network. Everything external looks at the class of the IP address instead of the subnet network ID. This way, traffic directed to your network externally still knows where to go, and once it has entered your internal network it can then be prioritized and segmented.

CIDR (classless Interdomain Routing)

CIDR is also known as classless routing or supernetting.

In CIDR conventional network class distinctions do not exist, a subnet boundary can move to the left, therefore generating more usable IP addresses on your network. A subnet created by moving the subnet boundary to the left is known as a supernet.

With CIDR also came new shorthand for denoting the position of subnet boundaries known as CIDR notation or slash notation. CIDR notation takes the form of the network ID followed by a forward slash (/) followed by the number of bits that are used for the extended network prefix.

To take advantage of classless routing, your networks routers must be able to interpret IP addresses that don;t adhere to conventional network class parameters. Routers that rely on older routing protocols (i.e. RIP) are not capable of interpreting classless IP addresses.

Internet Gateways

Gateways are a combination of software and hardware that enable two different network segments to exchange data. A gateway facilitates communication between different networks or subnets. Because on device cannot send data directly to a device on another subnet, a gateway must intercede and hand off the information. Every device on a TCP/IP based network has a default gateway (a gateway that first interprets its outbound requests to other subnets, and then interprets its inbound requests from other subnets).

The internet contains a vast number of routers and gateways. If each gateway had to track addressing information for every other gateway on the Internet, it would be overtaxed. Instead, each handles only a relatively small amount of addressing information, which it uses to forward data to another gateway that knows more about the data’s destination. The gateways that make up the internet backbone are called core gateways.

Address Translation

An organizations default gateway can also be used to “hide” the organizations internal IP addresses and keep them from being recognized on a public network.

A public network is one that any user may access with little or no restrictions.

On private networks, hiding IP addresses allows network managers more flexibility in assigning addresses. Clients behind a gateway may use any IP addressing scheme, regardless of whether it is recognized as legitimate by the Internet authorities but as soon as those devices need to go on the internet, they must have legitimate IP addresses to exchange data.

When a clients transmission reaches the default gateway, the gateway opens the IP datagram and replaces the client’s private IP address with an Internet recognized IP address. This process is known as NAT (Network Address Translation).

TCP/IP Mail Services

All Internet mail services rely on the same principles of mail delivery, storage, and pickup, though they may use different types of software to accomplish these functions.

Email servers and clients communicate through special TCP/IP application layer protocols. These protocols, all of which operate on a variety of operating systems are discussed below…

SMTP (Simple Mail transfer Protocol)

The protocol responsible for moving messages from one mail server to another over TCP/IP based networks. SMTP belongs to the application layer of the ODI model and relies on TCP as its transport protocol.

  • Operates from port 25 on the SMTP server
  • Simple sub-protocol, incapable of doing anything more than transporting mail or holding it in a queue

MIME (Multipurpose Internet Mail Extensions)

The standard message format specified by SMTP allows for lines that contain no more than 1000 ascii characters meaning if you relied solely on SMTP you would have very short messages and nothing like pictures included in an email.

  • MIME us a standard for encoding and interpreting binary files, images, video, and non-ascii character sets within an email message.
  • MIME identifies each element of a mail message according to content type.
  • MIME does not replace SMTP but works in conjunction with it.
  • Most modern email clients and servers support MIME

POP (Post Office Protocol)

  • POP is an application layer protocol used to retrieve messages from a mail server
  • POP3 relies on TCP and operates over port 110
  • With POP3 mail is delivered and stored on a mail server until it is downloaded by a user
  • Disadvantage of POP3 is that it typically does not allow users to save their messages on the server because of this IMAP is sometimes used

IMAP (Internet Message Access Protocol)

  • IMAP is a retrieval protocol that was developed as a more sophisticated alternative to POP3
  • The single biggest advantage IMAP4 has over POP3 is that users can store messages on the mail server, rather than having to continually download them
  • Users can retrieve all or only a portion of any mail message
  • Users can review their messages and delete them while the messages remain on the server
  • Users can create sophisticated methods of organizing messages on the server
  • Users can share a mailbox in a central location

Disadvantages of IMAP are typically related to the fact that it requires more storage space on the server.

Additional TCP/IP Utilities

Nearly all TCP/IP utilities can be accessed from the command prompt on any type of server or client running TCP/IP. The syntaxt may differ depending on the OS of the client.

Below is a list of additional TCP/IP utilities – research their use on your own!

  • Ipconfig (Windows) & Ifconfig (Linux)
  • Netstat
  • Nbtstat
  • Hostname, Host & Nslookup
  • Dig (Linux)
  • Whois (Linux)
  • Traceroute (Tracert)
  • Mtr (my traceroute)
  • Route


Some common operations include…

  • -a – Applies the command to all interfaces on a device, can be used with other switches
  • down – Marks the interface as unavailable to the network
  • up – Reinitializes the interface after it has been taken “down” so that it is once again available to the network


  • Stands for domain information groper.
  • Dig allows you to query a DNS database and find the host name associated with a specific IP address or vice versa.
  • Dig is useful for helping network administrators diagnose DNS problems
  • Dig can provide more information than nslookup
  • Dig is included with UNIX and Linux operating systems
Posted on Saturday, April 7, 2012 9:51 AM UNISA COS 2626 Networks | Back to top

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