PC ProSchools – TechTalk

Question: In DHCP when would I use reservations and when would I use exclusions?

Answer: Whenever you use static addresses on your network you should exclude them from your DHCP scope. That is exactly what exclusions are for, to stop DHCP from handing out addresses that you have already assigned statically. We use exclusions most often with servers and most servers will have a static address.

Address Reservations are used when you still want to hand out the IP address using DHCP but you want to make sure that the client always receives the same IP address. You will see this a lot with devices like network printers, most network printers are a pain in the neck to configure statically, it is easier to leave them to receive an IP address dynamically from DHCP and use a reservation to make sure they always receive the same IP address, this also means that if we have to change some information we can change it in the DHCP console instead of having to reconfigure the printer.

Question: Is a switch / hub the same as a router?

Answer: No, they are different.  A router is designed to connect two networks (or subnets) with completely different network IDs together, and route traffic between them.  Routers can offer many other features, such as performing Network Address Translation (NAT) and packet filtering.  They operate at Layer 3 of the OSI model.

Switches typically operate at Layer 2 of the OSI model.  They are typically designed to connect multiple devices together within a subnet.  These devices, for example, would be all of the computers in your classroom.  Switches are efficient in data transfer as well, and some have very cool features, like being Virtual-LAN (or V-LAN) capable.

Hubs are the most simplistic of the three.  They operate at Layer 1 of the OSI model and don’t really offer much for features.  They are an inexpensive way to connect multiple devices together on one subnet.  They are also not as efficient when transferring network data packets as switches, even though they look similar and perform the same basic functions.

Now some devices combine these technologies.  Wireless routers, for example offer routing capabilities, as well as act as switches for connecting a limited number of devices (4 or 5 usually) together.

Certification exams like Network+, offered by CompTIA, test you in these concepts.  However, they are excellent to research and learn about for any network administration role.  Google is your friend…

Question: I was under the impression that you could not have the same name twice in an AD Domain. But according to this, you can have two identical computer names in two separate DNS zones? Please explain this if I am wrong.

Answer: This is the beauty of the DNS hierarchy.  The same computer names can exist in different sections of the hierarchy.

The easiest example I can give you is www.  Take www.google.com and www.microsoft.com for example.  Both DNS namespaces use www as the host name (which is technically an alias, but whatever).  This would be a conflict on the same physical network, but you know that these web servers are not on the same physical network.

The reason this is possible is because to DNS, they exist in different places in the hierarchy.  They are both in the .com top level domain, but they are in different second level domains, Google and Microsoft respectively.  Because of this, there is no conflict.

Now think about how many www’s there are out on the Internet.  Thank God for DNS!

Active Directory databases can only have one name because it is the same database.  So, in one Active Directory domain, you can only have one object with that name; not two.

Additional Information: To muddy the waters a bit there is the addition of a new feature in Server 2008 DNS servers called the GlobalNames Zone.  In very basic terms it allows the resolution of single names (as opposed to FQDNs) throughout an entire AD Forest.  Designed to help organizations migrate away from WINS, it would actually require the use of unique names throughout every domain in an AD Forest.  Therefore, the following would not be possible;

A host named Computer1.contoso.com and another host in a child domain named Computer1.east.contoso.com Both computers have the GlobalNames name of Computer1 and name resolution using the GlobalNames zone would not be possible.  The solution would be to not use a GlobalNames zone at all.

Related Articles

• Student Question of the Week 1- Active Directory limitations.
• Read Only Domain Controller – RODC