Data communication part 6

Data CommunicationPart 6

Alex Fernandez

CALCULATING SUBNETS

◎ When you subnet a network ID, you need to follow the rules and conventions dictated by the good folks who developed TCP/IP to ensure that your new subnets can interact properly with each other and with larger networks. The rules to subnetting are as follows:

◎ 1. Starting with a beginning subnet mask, you extend the subnet extension until you have the number of subnets you need.

◎ 2. You cannot have an NE of all zeroes or all ones, so you calculate the number of subnets using this formula: new subnets = 2(number of network ID extension digits)– 2.

◎ 3. You cannot have a single-character network ID extension. You always start by moving the subnet at least two digits (Figure 7.26

◎ Since the network ID extension can’t be all zeroes and all ones, you only get two new network IDs.

◎ Figure 7.28 shows all of the IP addresses for each of the two new network IDs.

◎ Now convert these two network IDs back to dotted decimal:

◎ 192.168.4.64/26 (192.168.4.65 – 192.168.4.126)

◎ 192.168.4.128/26 (192.168.4.129 – 192.168.4.191)

◎ Congratulations! You’ve just taken a single network ID,

◎ 192.168.4.0/24, and subnetted it into two new network IDs!

◎ Figure 7.29 shows how you can use these two network IDs in a network.

◎ There’s only one problem the café needs three subnets, not just two! So let’s first figure out how large of ⎯a network ID extension is needed:

◎ ■ Two NE digits = 2 2

◎ – 2 = 2 network IDs

◎ ■ Three NE digits = 2

◎ 3 – 2 = 6 network IDs

◎ First, move the NE over three digits. This creates a /27 subnet for all the new network IDs (Figure 7.30).

◎ To help you visualize the address range, I’ll calculate the first two subnets—using 001 and 011 (Figure 7.31). Please do the other four for practice.

◎ Note that in this case you only get

◎ 25–2=30hosts per network

◎ ID! These better be small networks!

◎ Converting these to dotted decimal we get:

◎ 192.168.4.32/27 (192.168.4.33 – 192.168.4.62)

◎ 192.168.4.64/27 (192.168.4.65 – 192.168.4.94)

◎ 192.168.4.96/27 (192.168.4.97 – 192.168.4.126)

◎ 192.168.4.128/27 (192.168.4.129 – 192.168.4.158)

◎ 192.168.4.160/27 (192.168.4.161 – 192.168.4.190)

◎ 192.168.4.192/27 (192.168.4.193 – 192.168.4.222)

CIDR: SUBNETTING IN THE REAL WORLD

◎ there’s a better than average chance that you’ll never have to do subnetting in the real world. That’s not to say that subnetting isn’t important. It’s a critical part of the structure of the Internet.

◎ There are two situations in which subnetting most commonly takes place:

1. ISPs who receive class licenses from IANA and then subnet those class licenses for customers, and very large customers who take subnets (some -times already subnetted class licenses from ISPs) and make their own subnets.

2. even if you never do your own subnetting, there’s a pretty good

3. chance that you’ll contact an ISP and get CIDR addresses. You can’t think about subnet masks in terms of dotted decimal.

4. there’s a better than average chance you’ll look to more advanced IT certifications. Most Cisco, many Microsoft, and a large number of other certifications assume you understand subnetting.

USING IP ADDRESSES◎ There are two ways to give a computer an IP address,

subnet mask, and default gateway: either by typing in all the information (called static addressing) or by having some server program running on a system that automatically passes out all the IP information to systems as they boot up on or connect to a network (called dynamic addressing)

◎ Static IP Addressing

◉ Static addressing means typing all of the IP information into each of your clients. But before you type in anything, you have to answer two questions: what are you typing in and where do you type it? Let’s visualize a four-node network like the one shown in Figure 7.32.

◎ each computer must have an IP address, a subnet mask, and a default gateway.

◎ First, decide what network ID to use. In the old days, you were given a block of IP addresses from your ISP to use.

◎ Assume that’s still the method and you’ve been allocated a Class C license for 197.156.4/24.

◎ That

◎ said, most networks follow a common set of principles:

1. Give the default gateway the first IP address in the network ID.

2. Try to use the IP addresses in some kind of sequential order.

3. Try to separate servers from clients. For example, servers could have the IP addresses 197.156.4.10 to 197.156.4.19, while the clients range from 197.156.4.200 to 197.156.4.254.

4. Write down whatever you choose to do so the person who comes after you understands.

◎ IFCONFIG command