Simplified IP Addressing
We need to discuss netmasking, but first, let's digress for a moment. A Boolean AND is just like an “and” in English. You tell Johnny you will buy him an ice cream cone if he puts out the trash “and” makes his bed. If he does neither or only one of them, he doesn't get an ice cream cone. If he does both, he gets the cone.
Bitwise ANDs work bit by bit. So, if you AND a 1 with a 1, you get a 1. If you AND two 0s, a 1 and a 0, or a 0 and a 1, however, you get a 0. Table 4 illustrates this operation.
Now let's take a whole byte and do a Logical AND with another byte. Suppose the first byte is 10110010 and the second byte is 01100111. Working from the right, note that the first byte has a decimal value of
0*1 + 1*2 + 0*4 + 0*8 + 1*16 + 1*32 + 0*64 + 1*128 = 178
while the second byte has a decimal value of
1*1 + 1*2 + 1*4 + 0*8 + 0*16 + 1*32 + 1*64 + 0*128 = 103.Now, AND the two bytes:
1 0 1 1 0 0 1 0 178 decimal, ANDed with 0 1 1 0 0 1 1 1 103 decimal --------------- gives 0 0 1 0 0 0 1 0 34 decimalAs a second example, let's AND 178 with 255.
1 0 1 1 0 0 1 0 178 decimal, ANDed with 1 1 1 1 1 1 1 1 255 decimal --------------- gives 1 0 1 1 0 0 1 0 178 decimalWe know, then, that when you bit-wise AND any byte (number) with 255, you get the number dropping through, i.e., the result is merely the number again.
The default netmasks for the various classes are shown in Table 5 with some sample host IP addresses. Simply put, a host is anything that has an IP address. This includes servers, workstations, routers, etc.
So, what does this mean and what do we do with it? Let's work through Table 5. If we take the sample Class A address, 10.0.1.23 and bit-wise AND it with its default netmask, we obtain 10.0.0.0. What is 10.0.0.0? It's the network address—look at the last column.
Notice that the first byte gives the network address when ANDing a Class A network with its default netmask, while the first two bytes give the network address when ANDing a Class B IP address with the default Class B netmask. Hence, we say that the first byte of a Class A IP address gives the network address, and the three remaining bytes give the host addresses, i.e., a Class A address has the form N.H.H.H where N stands for Network and H stands for Host. Likewise, the first two bytes of a Class B IP address pertain to the network, and the last two bytes pertain to the host address, i.e., N.N.H.H. Finally, the first three bytes of a Class C IP address pertain to the network, while the last byte pertains to the host, i.e., N.N.N.H.
Let's illustrate this with a Class B IP address such as 22.214.171.124. From Table 5, we know that the default netmask for a Class B network is 255.255.0.0. Hence, ANDing the default mask with the IP address yields the address of the network that particular host is on, i.e., 126.96.36.199. So, a host with an IP address of 188.8.131.52 finds itself on a network with an IP address of 184.108.40.206 if a default Class B net-mask is used.
If you are granted a full Class B suite of addresses with a network address of 220.127.116.11, what do you do with them? Remember, a Class B network has the form of N.N.H.H, i.e., the last two bytes can be used for assigning host IP addresses. This yields a network with 2<+>16<+> - 2 host addresses. The -2 comes from the fact that 18.104.22.168 is the network address, so it can't be assigned to a host; the last address on the network, 22.214.171.124, is used for broadcasts, so it also can't be assigned to a host.
This would be a very big network (65,534 host addresses), far too big to be practical. A very simple approach is to “borrow” one byte's worth of host addresses and assign them as network addresses. That would yield 2<+>8<+> = 256 networks with 254 hosts on each. Even here, these are large networks. This process of borrowing host addresses and using them for networks is called subnetting. We accomplish this by using a sub-netmask (SNM). In this case, we would use a sub-netmask of 255.255.255.0, which is the default Class C netmask. Hence, we have taken one Class B network and turned it into 256 Class C networks.
If we AND 126.96.36.199 with 255.255.255.0, we get a network address of 188.8.131.52 with the first available host address of 184.108.40.206 and the last of 220.127.116.11, since 18.104.22.168 is reserved for broadcasts. Another way of doing this is to start with the network address (22.214.171.124 in this case), turn all host bits into 1s, and obtain the broadcast address. Here, the last byte is used for host addresses, so turning them to ones gives 126.96.36.199. This type of broadcast is called a directed broadcast, meaning that it jumps routers while a local broadcast (which doesn't jump routers) has the form 255.255.255.255 no matter which class of network is involved.
If you're not too stunned at this point, you may wonder if you can subnet only on byte boundaries or if you can subnet a Class C network. The answers are “no” and “yes”, respectively; i.e., you can work in the middle of a byte.
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
Free to Linux Journal readers.Register Now!
- SUSE LLC's SUSE Manager
- Murat Yener and Onur Dundar's Expert Android Studio (Wrox)
- My +1 Sword of Productivity
- Tech Tip: Really Simple HTTP Server with Python
- Managing Linux Using Puppet
- Non-Linux FOSS: Caffeine!
- Returning Values from Bash Functions
- Doing for User Space What We Did for Kernel Space
- Rogue Wave Software's Zend Server
- Google's SwiftShader Released
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide