DATA COMMUNICATION AND NETWORKS

DATA COMMUNICATION AND NETWORKS. UNIT 14: DOMAIN NAME SYSTEM & DEVICE DISCOVERY.

INTRODUCTION. The Domain Name System (DNS) handles the growing number of Internet users. DNS performs an important function on TCP/IP-based networks. DNS resolves hostnames, such as www.facebook.com, to IP addresses, such as 209.202.161.67. (IPv4 or IPv6 addresses) Such a resolution system makes it possible for people to remember the names of and refer to frequently used hosts using easy-to-remember hostnames rather than hard-to-remember IP addresses. DNS makes using Internet applications, such as the World Wide Web, easy. The process is as if, when phoning your friends and relatives, you could autodial them based on their names instead of having to remember their phone numbers..

DNS solves the problem of name resolution by offering resolution through servers configured to act as name servers. The name servers run DNS server software, which enables them to receive, process, and reply to requests from systems that want to resolve hostnames to IP addresses. Systems that ask DNS servers for a hostname-to-IP address mapping are called resolvers or DNS clients. In this example, the client asks to reach the first server at mycoltd.com; the router turns to the DNS server for an IP address associated with that server; and after the address is returned, the client can establish a connection. By default, DNS operates on port 53. Note: Like other TCP/IP-based services, DNS is a platform-independent protocol. Therefore, it can be used on Linux, UNIX, Windows, and almost every other platform..

[image] serverl.mycoltd.com IP address of server serverl.mycoltd.com is 192.168.3.43.

HOW DNS WORKS. To understand how DNS works, imagine a typical user, John, logging in to his computer. He launches his web browser so that he can view the website at a company. He enters the name of their website http://www.example.com. Then the process below follows: 1. John’s workstation sends a request to the DNS server about the IP address of www.example.com. 2. The DNS server checks its database to find that www.example.com corresponds to 192.168.1.4. 3. The server returns this address to John’s browser. 4. The browser uses the address to locate the website. 5. The browser displays the website on John’s monitor..

Because the DNS namespace is large, a single server cannot hold all the records for the entire namespace. As a result, there is a good chance that a given DNS server might not resolve the request for a certain entry. In this case, the DNS server asks another DNS server if it has an entry for the host. One of the problems with DNS is that, despite all its automatic resolution capabilities, entries and changes to those entries must still be manually performed. A strategy to solve this problem is to use Dynamic DNS (DDNS), a newer system that enables hosts to be dynamically registered with the DNS server. By making changes in real-time to hostnames, addresses, and related information, there is less likelihood of not finding a server or site that has been recently added or changed..

The DNS Namespace. DNS operates in the DNS namespace. This space has logical divisions hierarchically organized. At the top level are domains such as .com (commercial) and .edu (education), as well as domains for countries, such as .uk (United Kingdom) and .de (Germany). Below the top level are subdomains or second-level domains associated with organizations or commercial companies, such as Red Hat and Microsoft. Within these domains, hosts or other subdomains can be assigned. For example, the server ftp.redhat.com would be in the redhat.com domain. Note: The domain name, along with any subdomains, is called the fully qualified domain name (FQDN) because it includes all the components from the top of the DNS namespace to the host. An example of an FQDN is www.comptia.org, where www is the host, CompTIA is the second-level domain, and .org is the top-level domain..

Figure 2: shows a DNS hierarchical namespace.. [image] ur puooKw asnOH311HM A09 EOSOHOIS,N woo•jeqpordu IVH03U noo uoddns anov.

The lower domains are largely open to use in whatever way the domain name holder sees fit. However, the top-level domains are relatively closely controlled. Recently, a number of top-level domains were added, mainly to accommodate the increasing need for hostnames..

NOTE: In addition to country-specific domains, many countries have created subdomains that follow roughly the same principles as the original top-level domains (such as co.uk and gov.nz). DNS was designed as a tree structure, where each named domain is a node in the tree. The top-most node of the tree is the DNS root domain (.), under which there are subdomains, such as .com, .edu, .gov, and .mil Although the assignment of domain names is supposed to conform to the structure shown in Table 1, the assignment of names is not as closely controlled as you might think. It’s not uncommon for some domain names to be used for other purposes, such as .org or .net being used for business. Although the primary function of DNS is to resolve hostnames to IP addresses, you can also have DNS perform IP address-to-hostname resolution. This process is called reverse lookup, which is accomplished by using pointer (PTR) records..

Types of DNS Entries. Although the most common entry in a DNS database is an A (address) record, which maps a hostname to an IP address, DNS can hold numerous other types of entries as well. Some are the MX record, which can map entries that correspond to mail exchanger systems, and CNAME (canonical record name), which can create alias records for a system. A system can have an A record and then multiple CNAME entries for its aliases. A DNS table with all these types of entries might look like this: fileserve.mycoltd.com IN A 192.168.33.2 email.mycoltd.com IN A 192.168.33.7 fileprint.mycoltd.com IN CNAME fileserver.mycoltd.com mailer.mycoltd.com IN MX 10 email.mycoltd.com As you can see, rather than map to an actual IP address, the CNAME and MX record entries map to another host, which DNS in turn can resolve to an IP address..

DNS Records. Each DNS name server maintains information about its zone, or domain, in a series of records, known as DNS resource records. There are several DNS resource records; each contains information about the DNS domain and the systems within it. These records are text entries stored on the DNS server. Some of the DNS resource records include the following: Start of Authority (SOA): A record of information containing data on DNS zones and other DNS records. A DNS zone is the part of a domain for which an individual DNS server is responsible. Each zone contains a single SOA record. Name Server (NS): Stores information that identifies the name servers in the domain that store information for that domain. Service Locator (SRV): This is a generalized service location record, used for newer protocols instead of creating protocol-specific records such as MX..

4) Canonical Name (CNAME): Stores additional hostnames, or aliases, for hosts in the domain. A CNAME specifies an alias or nickname for a canonical hostname record in a Domain Name Service (DNS) database. CNAME records give a single computer multiple names (aliases). 5) Pointer (PTR): A pointer to the canonical name, which is used to perform a reverse DNS lookup, in which case the name is returned when the query originates with an IP address. 6) IPv6 Address (AAAA): Stores information for IPv6 (128-bit) addresses. It is most commonly used to map hostnames to an IP address for a host. 7) IPv4 Address (A): Stores information for IPv4 (32-bit) addresses. It is most commonly used to map hostnames to an IP address for a host. 8) Text (TXT): This field was originally created to carry human-readable text in a DNS record, but that purpose has long since passed. Today, it is more common that it holds machine-readable data, such as SPF (Sender Policy Framework), and DKIM (DomainKeys Identified Mail). 9) Mail Exchange (MX): Stores information about where mail for the domain should be delivered. Note: The most common type of DNS zone is the forward lookup zone, which allows DNS clients to obtain information such as IP addresses that correspond to DNS domain names. Remember that a reverse lookup zone maps from IP addresses back to DNS domain names..

In a real-world scenario, whether you use DNS is almost a nonissue. If you have Internet access, you will most certainly use DNS, but you are likely to use the DNS facilities of your Internet service provider (ISP) rather than have your own internal DNS server this is known as external DNS. However, if you operate a large, complex, multiplatform network, you might find that internal DNS servers are necessary. The major network operating system vendors know that you might need DNS facilities in your organization, so they include DNS server applications with their offerings, making third-party/cloud-hosted DNS a possibility. Google, for example, offers Cloud DNS, which is “low latency, high availability and is a cost-effective way to make your applications and services available to your users”. It is common practice for workstations to be configured with the IP addresses of two DNS servers for fault tolerance (configured via the Alternate Configuration tab in Windows, for example). The importance of DNS, particularly in environments in which the Internet is heavily used, cannot be overstated. If DNS facilities are not accessible, the Internet effectively becomes unusable, unless you can remember the IP addresses of all your favorite sites..

Windows Internet Name Service (WINS). On Windows networks, you can use a system called WINS to enable Network Basic Input/Output System (NetBIOS) names to be resolved to IP addresses. NetBIOS name resolution is necessary on Windows networks so that systems can locate and access each other by using the NetBIOS computer name rather than the IP address. It is a lot easier for a person to remember a computer called secretary than to remember its IP address, 192.168.2.34. The NetBIOS name needs to be resolved to an IP address and subsequently to a MAC address (by ARP). NetBIOS name resolution can be performed three ways on a network. The simplest way is to use a WINS server on the network that automatically performs the NetBIOS name resolution..

If a WINS server is not available, NetBIOS name resolution can be performed statically using an LMHOSTS file. Using an LMHOSTS file requires that you manually configure at least one text file with the entries. As you can imagine, this can be a time-consuming process, particularly if the systems on the network frequently change. The third method, and the default, is that systems resolve NetBIOS names using broadcasts. This approach has two problems. First, the broadcasts create additional network traffic, and second, the broadcasts cannot traverse routers unless the router is configured to forward them. This means that resolutions between network segments are impossible..

Device discovery.

Module Title: Address Resolution Module Objective: Explain how ARP and ND enable communication on a network. Source: CISCO (CCNA 1).