Basic Network Topology
This chapter provides a common and basic network topology as an example for the ASDM Startup Wizard procedure. The topology is such a standard template that, unless you have a unusual circumstance, you can follow these steps verbatim and have secure Internet connectivity upon completion.
Figure 5-1 shows the topology you will use for your ASDM Startup Wizard procedure.
It's important to understand the basic elements and terminology used in this topology:
- Service provider
- Default gateway
- IP address and subnet masks
- Inside address
- Inside network
- Internet server addresses
Figure 5-1 Basic Network to Internet Topology
Understanding the Elements of Your Network
Before you begin, let's review some basic Internet terminology to ensure that we are all on the same page.
The Internet, which of course everyone is familiar with, is millions of networks and hosts that reach all over the globe. Most companies want to be connected to the Internet to either tap into this vast array of information, communicate with others who have Internet access, or provide services or advertisement to other Internet users.
The Internet Service Provider
The Internet service provider (ISP) is your entry point into the Internet. There are many different providers throughout the world. Your first step to connect to the Internet is to select a local ISP. Your local ISP might be a local telephone company or a small company that focuses on providing Internet coverage in small target areas. It is beyond the scope of this book to recommend ISPs. It is recommended, however, that you talk to other companies or use the web to find out who provides the best service in your area.
The ISP provides you with a physical point of entry into its network. This is usually a networking device such as a DSL/cable modem or a router. The outside of your ASA/PIX Security Appliance plugs into this device.
Your ISP will provide an IP address called a default gateway address. This is the address of a router that the ISP owns that will be the next hop toward the Internet from your ASA/PIX Security Appliance.
Internet IP Address and Subnet Mask
Along with a default gateway, the ISP will provide you with an IP address and a subnet mask. The IP address and subnet mask are applied to the outside of your firewall. The IP address always has four octets and might look something like 220.127.116.11. The address and mask work together to form a network number and a host number. You don't have to worry about this for now; suffice it to say that if you correctly enter your IP address and mask, you will be properly connected to the Internet.
The concept of inside addresses could get complex if you let it. Don't let it! It can also be simple. Addresses on the inside of your network are usually defined as private or reserved addresses.
Don't be intimidated if you don't understand IP addressing. If you enter the addresses correctly from your ISP and follow the procedures in this book, the details of IP addressing are insignificant. The most commonly used private addresses are the following:
- Class A: 10.0.0.0 through 10.255.255.255 (subnet mask: 255.0.0.0)
- Class B: 172.16.0.0 through 172.31.255.255 (subnet mask: 255.255.0.0)
- Class C: 192.168.0.0 through 192.168.255.255 (subnet mask: 255.255.255.0)
It doesn't matter which of these addresses you elect to use because they all function in exactly the same manner. In this book, you use 192.168.1.x and a subnet mask of 255.255.255.0 as your private inside network addresses.
Network Address Translation
One thing you must understand about private inside addresses is that they are not routable on the Internet. This means that if you were to send traffic out from a host that has a private address to http://www.cisco.com, the packet would make it out to the Internet. However, because it's a private nonroutable Internet address, it cannot be returned to the host that originated the request. Therefore, your web request would never be returned and your connection would time out.
The ASA/PIX Security Appliance handles this situation by using a concept called network address translation (NAT). When NAT is applied on the firewall, it uses the following process to ensure that the traffic is both delivered to the destination address and then returned back to the host that has the private address:
- Looks at the address of the host requesting Internet access
- Stores the original private address in its NAT lookup table
- Replaces the private address with either the outside interface IP address of your ASA/PIX Security Appliance or another address provided by your service provider
- Accepts the return packet, looks up the host that made the original request in its NAT table, and replaces the private address
- Routes the return packet back to the correct host
The inside network refers to the hosts that will be behind your firewall. Normally, these hosts are connected to a switch or a series of switches and routers, depending on the complexity of your network. Because this book is a beginner's guide, you are going to use the topology shown in Figure 5-1, which is a flat network, which means that the inside of your firewall and all of your hosts will be connected to a single switch comprising your inside network.
Your inside network addresses must be in the same subnet as the inside interface of your ASA/PIX Security Appliance. You could manually configure the IP addresses of each of your inside hosts so that they are in the same subnet. However, the security appliance can also do this for you. You will use a function of the ASA/PIX Security Appliance called Dynamic Host Configuration Protocol (DHCP) to automatically assign IP addresses to your inside host. This protocol will save you many hours of initial labor and ongoing maintenance.
Public Servers: Mail DNS, Web Servers
Public servers provide services that you plan to allow Internet users to access. The most common of these are web servers, mail servers, and DNS servers. If you plan to offer public services, you must request an IP address for each of the servers from your ISP. Also, remember from earlier reading, that if you are going to offer public services, you need to have a firewall (such as a PIX 515E) that has more than two interfaces. This way, you can put your servers on an interface separate from your inside users. This setup ensures that if hackers do compromise one of your public servers, your inside network is still isolated from the hackers. Chapter 6 covers the deployment of these servers.