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DNS and DHCP Explained

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Introduction

This lesson will describe Domain Name Service (DNS), Dynamic Host Configuration Protocol (DHCP) as well as other protocols and port relationships.

Lesson Objectives

By The End Of This Lesson, You Will Be Able To:

  1. Explain the purpose of DNS.
  2. Explain the purpose of DHCP.
  3. Identify common ports and networking protocols.

Domain Name Service (DNS)

Domain Name Service (DNS) is a hierarchical database for computers, services, or any resource connected to the Internet or a private network.

Humans can remember domain names (i.e. www.google.com and www.yahoo.com), but not their associated IP addresses. (i.e. 74.125.255.244, 206.190.36.45). DNS servers are responsible for resolving Fully Qualified Domain Names (FQDN’s) into their associated IP addresses. Websites are hosted on web servers which have FQDN’s.

There are Top Level Domain (TLD) Root Servers responsible for resolving queries of lower level domains. TLD’s including: .com, .net, .org, .mil, .edu, and .gov. DNS servers maintain a database of records to efficiently resolve names (FQDN and URL’s) to IP addresses. URL is short for Uniform Resource Locator.

These resource records include the following types:

  • A (Address Record)

Maps a host to an IP address for example: 172.34.2.10 ‘hostA’.

  • AAAA (IPv6 Address Record)

Maps a host to an IP address in IPv6 for example: fe80:140b:5771:hb18:bd22:1144:v57r:1234 ‘hostA’.

  • MX (Mail Exchange Record)

Identifies which server within the domain/network is responsible for managing incoming email.

  • CNAME (Canonical Name Record)

This record type provides an alias for a domain name.

  • PTR (Pointer Record)

Maps an IP address to a host for example: 10.2.34.172. in-addr.arpa ‘hostA’). With PTR records DNS is resolving for the IP Address not the domain name as with the A record.

Think About

I know the IP address what is the domain name?

‘A’ records are forward lookup zones and ‘PTR’ records are reverse lookup zones. The local host file on a Windows computer maps hostnames to IP address just like DNS. C:\Windows\System32\drivers\etc\host

Dynamic DNS

A record can change if a host was moved from one network to another or if the IP address is dynamic (changing).

Dynamic DNS (DDNS) enables the host systems to automatically send updates to the authoritative DNS when their records have changed. DDNS updates can be local or global. DDNS provides a more efficient process to maintain real time DNS records.

Dynamic Host Configuration Protocol (DHCP)

Configuring static IPs can create an IP management problem. Administrator going to every machine and typing in an IP address or if there is a duplicate IP address finding which machine needs to be changed.

DHCP provides the ability to have host machines query a server for a dynamically assigned IP address.

DHCP requires a server and a client. The client is configured on the host machine (i.e. Windows 7). Static IP addresses can be manually set on the client, but requires high overhead maintenance.

An administrator can:

• Reserve an IP address to be assigned to specific host (reservations).

• Define the range of IP addresses that can be given out (scope).

• Specify the time a host can use a dynamically assigned address before being assigned a new one (lease).

• Specify the DNS Server and suffix (i.e. adatum.com) within the DHCP server.

Ports

Say that I know the location of the system I want to communicate with, how do I obtain access to this location? The answer is ports. Remember Application refers to the protocol that enables communication (i.e. A webserver uses the Application Level protocol (HTTP – port 80) to communicate with a client).

There are two protocols, Transmission Control Protocol (TCP) which is connection orientated, and User Datagram Protocol (UDP) which is connectionless. These communication protocols use different ports based on the type of application being used (i.e. DNS).

Internet Corporation for Assigned Names and Numbers (ICAAN) – is the organization responsible for the coordination of unique identifiers (i.e. ports) on the internet.

There are a total of 65,535 ports, separated into three groups:

  • Well – Known ports (1 – 1023)
  • Registered ports (1024 – 49,151)
  • Dynamic or Private ports (49,152 – 65,535)

Example Port Protocols

Here is a list of some of the more common protocols and their Transmission Control Protocol (TCP) or UDP ports:

    • Simple Mail Transport Protocol (SMTP) – 25 (TCP)
    • HyperText Transfer Protocol (HTTP) – 80 (TCP)
    • HyperText Transfer Protocol Secure (HTTPS) – 443 (TCP)
    • File Transfer Protocol (FTP) – 20, 21 (TCP)
    • TELNET – 23 (TCP)
    • Internet Message Access Protocol (IMAP) – 143 (TCP)
    • Remote Desktop Protocol (RDP) – 3389 (UDP & TCP)
    • Secure Shell (SSH) – 22 (TCP)
    • Domain Name System (DNS) – 53 (UDP & TCP)
    • Dynamic Host Configuration Protocol (DHCP) – 67, 68 (UDP)

What are these protocols used for? Here is a list of examples.

  • File Transfer Protocol (FTP): FTP transfers large files from one computer system to another. Authentication is required.
  • Trivial File Transfer Protocol (TFTP): TFTP transfers small amounts of data within private networks. Authentication not required.
  • TELNET provides remote command execution between two networked systems.
  • Secure Shell (SSH): SSH provides secure remote command execution between two network systems through encryption.
  • Transport Layer Security (TLS): TSL provides secure communication over the internet.
  • Hpertext Transfer Protocol (HTTP): HTTP is used to communicate (i.e. exchange and transfer data) on the Internet.
  • Hpertext Transfer Protocol Secure (HTTPS): HTTPS uses SSL/TLS for encryption on top of HTTP.
  • Domain Name System (DNS): DNS translates URL’s and domain names into IP addresses.
  • Address Resolution Protocol (ARP): ARP provides the method for identifying which MAC address matches a known IP address.
  • Simple Mail Transfer Protocol (SMTP): SMTP used to send and receive email from source and destination mail servers.
  • Post Office Protocol (POP3): POP3 used by email clients (i.e. Outlook) to retrieve mail from server (i.e. Exchange)
  • Internet Message Access Protocol (IMAP4): IMAPA4 is used by mail clients (i.e. Eudora, Entourage) to retrieve mail from server (i.e. Apple MailServer).
  • Dynamic Host Configuration Protocol (DHCP): DHCP is used by hosts to obtain IP address from DHCP server.
  • Network Time Protocol (NTP): NTP synchronizes time of internal clocks within systems on a network.
  • Internet Control Message Protocol (ICMP): ICMP is used to send status information between two or more systems on a network.
  • Internet Group Management Protocol (IGMP): IGMP is used to send multicast communication between hosts.
  • Simple Network Management Protocol (SNMP2/3): SNMP2/3 provides a method for managing and gathering information from devices on a network.
  • Session Initiated Protocol (SIP): SIP handles communication sessions of voice and video calls over IP (VoIP).
  • Real-time Transport Protocol (RTP): RTP provides the format for packets sent relating to voice and video calls over IP (VoIP).

Summary

In this lesson there was a lot of information covered so here is a brief recap.

Following how the physical internal (BUS) components of a workstation interact with the Network Interface led to various dependent topics including operating systems, data packets, MAC & IP addresses, access methods, subnet mask, subnetting, DNS, and port assignments.

DHCP is used within networks to manage IP address allocation. Likewise, TCP and UDP ports manage communication between application programs.

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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.