ITEC 275 Computer Networks – Switching, Routing, and WANs. Week 7 Robert D’Andrea. Some slides provide by Priscilla Oppenheimer and used with permission. Agenda. Learning Activities Domain Name Server (DNS) Summarization Root Owner DNS Routing tables Spanning Tree Protocol
Some slides provide by Priscilla Oppenheimer and used with permission
DNS has a method of noting and interpreting the fully qualified path to a DNS domain name similar to the way full paths to files or directories are noted or displayed at a command prompt.
For example, a directory tree path helps point to the exact location of a file stored on your computer. For Windows computers, the back slash (\) indicates each new directory that leads to the exact location of a file. For DNS, the equivalent is a period (.) indicating each new domain level used in a name.
UNIX uses the concept of relative and absolute file names. If a file name is preceded by a forward slash (/bin), the name is absolute. If the name is without a leading slash, it is considered relative to your current working directory.
For DNS, an example of a domain name with multiple levels is the following, a fully qualified domain name (FQDN):
Unlike the file name example, a DNS FQDN, when read from left to right, moves from its most specific information (the DNS name for a computer called "host-a") to its highest or most general piece of information (the trailing period (.) that indicates the root of the DNS name tree). This example shows the four separate DNS domain levels that lead away from the specific host location of "host-a":
1. The "example" domain, which corresponds to a subdomain where the computer name "host-a" is registered for use.
2. The "microsoft" domain, which corresponds to the parent domain that roots the "example" subdomain.
3. The "com" domain, which corresponds to the top-level domain designated for use by business or commercial organizations that roots the "microsoft" domain.
4. The trailing period (.), which is a standard separator character used to qualify the full DNS domain name to the root level of the DNS namespace tree.
When a computer on the Internet needs to resolve a domain name, it uses resolver software to do the lookup. A resolver breaks the name up into its labels from right to left. The first component is queried using a root server to obtain the responsible authoritative server. Queries for each name are performed until a name server returns the answer of the original query.
As of 2013, there are 13 root name servers, with names in the form letter.root-server.net. This does not mean that there are only 13 phsyical servers; each site uses redundant computer equipment to provide reliable service in when hardware and software fail on occasion.
X = blocked by STP
Autonomous system (two definitions that are often used):
“A set of routers that presents a common routing policy to the internetwork”
“A network or set of networks that are under the administrative control of a single entity”
RouterA(config)#ip route 172.16.50.0 255.255.255.0 172.16.20.2
Send packets for subnet 50 to 172.16.20.2 (Router B)
RouterA(config)#ip route 0.0.0.0 0.0.0.0 172.16.20.2
If it’s not local, send it to 172.16.20.2 (Router B)
Router A’s Routing Table
Router B’s Routing Table
Network Distance Send To
172.16.0.0 0 Port 1
192.168.2.0 1 Router B
Network Distance Send To
192.168.2.0 0 Port 1 172.16.0.0 1 Router A
Area 0 (Backbone)