I'm learning about networking, at home I have two physical machines and a bunch of VMs that I use to test my applications each machine has a different hostname and I map them manually in each /etc/hosts files  I would like to know what's the difference between home IP adress (127.0.0.1) and a real IP address given by the network in /etc/hosts  for example   let's say my IP address is 192.168.2.20 and the name host is naruto and my /etc/hosts looks like this:  127.0.0.1       localhost 192.168.2.20    naruto 127.0.0.1       naruto   all lines point to the same machine I understand that the main difference is how programs connect to each of them two are using loopback device and the other one is using a nic. my question is should I have all these lines? or what lines should I have? what's the use of each of them?  I was reading this  but it didn't help, I got more confused
As I understand it, the hosts file is one of several system facilities that assists in addressing network nodes in a computer network. But what should be inside it? When I install Ubuntu by default   127.0.0.1 localhost    will be there. Why?   How does /etc/hosts work in case of JVM systems like Cassandra? When is DNS alternative, I guess not on a single computer?
Induced metric on a null hypersurface Deriving the inverse of a 2x2 matrix I am looking for a derivation for the inverse of a 2x2 matrix. I am also wondering why the determinant is involved in the expression. I am familiar with high school maths and linear algebra. If there is an intuitive reason for expression i would also be interested in that. Why does getopts only work the first time? I want to install the latest Firefox on my Ubuntu (64bit) box.  What is the best way to do this? Is there a specific update site or should I download the binaries manually? If doing manually, in which directory should I put the files? In my home directory, or is there a better place? How can I make my IP address appear to be from another country? How do I make it appear that my IP address is coming from one country while I'm located in another? I live in Germany and some websites (like Hulu or Youtube) don't work because my IP isn't in the US. How do I get around this? Do I have to use a proxy or something?  Moderator note Super User does not endorse nor defend any activity which may be used to circumvent local/state/national laws. Determinant of a non-square matrix I wrote an answer to  question based on determinants, but subsequently deleted it because the OP is interested in non-square matrices, which effectively blocks the use of determinants and thereby undermined the entire answer. However, it can be salvaged if there exists a function $\det$ defined on all real-valued matrices (not just the square ones) having the following properties.   $\det$ is real-valued $\det$ has its usual value for square matrices $\det(AB)$ always equals $\det(A)\det(B)$ whenever the product $AB$ is defined. $\det(A) \neq 0$ iff $\det(A^\top) \neq 0$   Does such a function exist? Sorry to start a subjective question, but want to understand "own.sty" file.  Other than all \usepackage{} what sort of other commands can be added to the makeyourown.sty file.  Can you include \let command, and \newcommands in it.   What sort of precaution / structure newbies should follow. Switch statement fall-through...should it be allowed? How did "When" become the customary answer to "say when"? When a waiter at a restaurant comes by with pepper or Parmesan cheese, he says, "say when" and starts putting it on your food.  Many people will say "OK" or "that's enough," but it seems that the customary answer is "when".  How did this become the customary answer?  Did everyone decide to make the exact same joke, until it became commonplace.  I thought it was funny when I was a kid, but even now as an adult, I still say it.  For evidence that this is indeed commonplace, see .  The answer "when" is used and is external to the joke of the strip. But even so, I'm curious as to just how commonplace this really is.  I see it a lot in the US.  Is it used elsewhere in the world?  (by the way, I'm new to asking questions on this site, and I have no idea what tags to put on this question) Dirichlet's problem in the unit disk is to construct the harmonic function from the given continuous function on the boundary circle. It is solved by the convolution with the Poisson kernel, and we know that the constructed function has uniform limit which is the given continuous function as $r\to 1$, and we can complexify the solution by taking the Schwarz integral formula. Now (in this very particular case) does the imaginary part has a uniform limit? If it does, why don't we prove the uniqueness of the solution by a simple application of the Cauchy integral formula? Here is the proof that I reckon to be reasonable(which is what I expose here to be criticised)  We construct the Schwarz integral, or $\frac{1}{2\pi}\int _{0}^{2\pi}\frac{\exp(it)+z}{\exp{(it)}-z}u(\exp{(it)})\mathrm{d}t$. This function is an analytic function in the interior unit disk, and is continuous in the closed unit disk(its closure). Let this analytic function denoted by $f$, and another analytic function that has the same uniform limit function on the circle be $v$. Then $u-v$ is an complex analytic function in the interior of the unit disk, continuous on the closure, with boundary value identically zero. An application of Cauchy's integral formula gives rise to the conclusion that $u-v$ in identically zero on the entire disk.   Another question being if the boundary function has finite jump discontinuities, what is the behaviour of the convolution with the Poisson kernel near the discontinuous points? Method always returns true? Given a polynomial over a field, what are the methods to see it is irreducible? Only two comes to my mind now. First is Eisenstein criterion. Another is that if a polynomial is irreducible mod p then it is irreducible. Are there any others? Is globbing a feature of the shell? Algorithm to detect intersection of two rectangles? I'm looking for an algorithm to detect if two rectangles intersect (one at an arbitrary angle, the other with only vertical/horizontal lines).  Testing if a corner of one is in the other ALMOST works.  It fails if the rectangles form a cross-like shape.  It seems like a good idea to avoid using slopes of the lines, which would require special cases for vertical lines. Intuition for $\,c\mid a,b \iff c\mid \gcd(a,b)$ Russian babel and chemformula Strictly diagonally dominant matrices are non singular Why I can't remove linux partitions on macOS Sierra I'm trying to remove linux partition from my iMac (21.5-inch, Mid 2011) on MacOS Sierra 10.12 Beta 2, So I tried existing solutions by using Disk Utility, I tried from Recovery as well, But it says      You can not remove this volume because the previous volume can not be resized         Some  use Terminal but I have no idea about that.  I need a simplest and easiest way to remove these linux partitions and grow my mac partition. How to choose a lens for my first DSLR? I am considering buying my first DSLR.  How do I know whether I need a 18-200 lens, 17-85, or 18-55? Or something else? Proving that $AB-BA=cI$ for nontrivial $c \in \mathbb{C}$ I have a homework question I can`t solve:  Let $X$ be a normed linear space, $A,B \in B(X)$. Show that there exists no nontrivial $c \in \mathbb{C} $ such that $AB-BA=cI$.  Thanks alot already guys! I seem to have a professor who gives impossible homework problems.  The hint he gave: $A^{n}B-BA^{n}=ncA^{n-1}$, $n \in \mathbb{N}$