Cisco® CCNA Exam Cram Notes : DNS(Domain Name Service )

VII. IP connectivity and Services

1. DNS(Domain Name Service )

DNS resolves Domain names with IP addresses. WINS resolves NetBIOS names to IP addresses. The core administrative unit in DNS is called a zone. A zone is a physical file composed of resource records that define a group of domains. A domain is a node in the DNS name space and all sub-domains below it.

Any web server can be made to be present on the public Internet by using an external DNS name and a public IP address. Of course, you need to ensure physical connectivity to the Internet, and a broadband router with ISP connection is a good solution for this. Web server listens to port 80, therefore, you need to enable port 80 (incoming) on the router.

When a Cisco router is initially configured, the router's DNS lookup function is enabled by default. This function is only useful if the router utilizes a DNS server on the network otherwise, it will cause delays to users. When an erroneous URL is typed, the DNS lookup function will attempt to find the URL on the DNS server. If no DNS server is available, the user's computer will hang while the lookup is performed. To decrease user delays if no DNS server is configured, disable the DNS lookup function on the Cisco router.

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