Questions tagged [networking]

Networking refers to the technologies and techniques that enable the interconnection of devices and applications allowing them to communicate electronically.

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476 votes
8 answers

How does IPv4 Subnetting Work?

This is a Canonical Question about IPv4 Subnets. Related: How does IPv6 subnetting work and how does it differ from IPv4 subnetting? How does Subnetting Work, and How do you do it by ...
268 votes
15 answers

I've inherited a rat's nest of cabling. What now?

You know, you see pictures like below and sort of chuckle until you actually have to deal with it. I have just inherited something that looks like the picture below. The culture of the organization ...
Chad Harrison's user avatar
229 votes
3 answers

What is the difference between Unix sockets and TCP/IP sockets?

It's from this answer: When the host is "localhost", MySQL Unix clients use a Unix socket, AKA Unix ...
apache's user avatar
  • 3,297
217 votes
8 answers

Setting the hostname: FQDN or short name?

I've noticed that the "preferred" method of setting the system hostname is fundamentally different between Red Hat/CentOS and Debian/Ubuntu systems. CentOS documentation and the RHEL deployment guide ...
Cakemox's user avatar
  • 25.4k
201 votes
6 answers

VirtualBox: How to set up networking so both host and guest can access internet and talk to each other

I was wondering if someone could give me a simple guide on how to set up virtual networking in VirtualBox (4.0.2) so that the following scenarios work: Both Host and Guest can access the Internet ...
Svish's user avatar
  • 7,117
190 votes
4 answers

Should I quit using Ifconfig?

With the servers that mount Infiniband cards, when I use the ifconfig command, I get this warning: Ifconfig uses the ioctl access method to get the full address information, which limits hardware ...
Zhen's user avatar
  • 2,159
189 votes
9 answers

Resolve host name from IP address

I'm looking for a command line tool which gets an IP address and returns the host name, for Windows.
DouglasJose's user avatar
  • 2,013
187 votes
13 answers

Why couldn't MAC addresses be used instead of IPv4|6 for networking? [closed]

I am reading up on TCP/IP and other related protocols and technologies. MAC addresses are described as being (reasonably :) unique, and as having a large possibility space (several hundred trillions), ...
Félix Saparelli's user avatar
155 votes
2 answers

What causes the 'Connection Refused' message?

This is a Canonical Question about Connection Refused We see a lot of questions to the effect When I try to connect to a system I get a message Connection refused Why is this ?
user9517's user avatar
  • 116k
151 votes
2 answers

How do VLANs work?

What are VLANs? What problems do they solve? I'm helping a friend learn basic networking, as he's just become the sole sysadmin at a small company. I've been pointing him at various questions/answers ...
Murali Suriar's user avatar
145 votes
8 answers

In Windows, using the command line, how do you check if a remote port is open?

What is a simple way in Windows to test if traffic gets through to a specific port on a remote machine?
Matt's user avatar
  • 1,631
143 votes
6 answers

Why do consoles sometimes hang forever when SSH connection breaks?

I've seen this with so many consoles (on Linux, Mac, ...), and with lots of different machines in many different networks. I can never pinpoint the exact reason, why this happens: All you have to do ...
Chris Lercher's user avatar
137 votes
5 answers

How does IPv6 subnetting work and how does it differ from IPv4 subnetting?

This is a Canonical Question about IPv6 Subnetting. Related: How does IPv4 Subnetting Work? I know a lot about IPv4 Subnetting, and as I prepare to (deploy|work on) an IPv6 network I ...
Michael Hampton's user avatar
132 votes
8 answers

How to forcibly close a socket in TIME_WAIT?

I run a particular program on linux which sometimes crashes. If you open it quickly after that, it listens on socket 49201 instead of 49200 as it did the first time. netstat reveals that 49200 is in ...
Rehan's user avatar
  • 1,423
128 votes
17 answers

Switching to IPv6 implies dropping NAT. Is that a good thing?

This is a Canonical Question about IPv6 and NAT Related: How does IPv6 subnetting work and how does it differ from IPv4 subnetting? How can I 'dip my toes' into dynamic IPv6 network ...
Ernie's user avatar
  • 5,352
126 votes
4 answers

How to check if a port is blocked on a Windows machine?

On the Windows platform, what native options to I have to check if a port (3306, for example) on my local machine (as in localhost), is being blocked?
Boris Pavlović's user avatar
120 votes
5 answers

What is this IP address:

I have always noticed an IP something "169.254.x.x" in my routing table even when I am not connected to any network in my Windows operating system. In Linux, when I list my routing table. $ ...
pradeepchhetri's user avatar
119 votes
9 answers

How much network latency is "typical" for east - west coast USA?

At the moment we're trying to decide whether to move our datacenter from the west coast to the east coast. However, I am seeing some disturbing latency numbers from my west coast location to the east ...
Jeff Atwood's user avatar
  • 13.2k
109 votes
16 answers

How do I find if there is a rogue DHCP server on my Network?

What's the best approach towards determining if I have a rogue DHCP server inside my network? I'm wondering how most admins approach these kinds of problems. I found DHCP Probe through searching, ...
l0c0b0x's user avatar
  • 12k
107 votes
6 answers

Is X.Y.Z.0 a valid IP address?

Are IP addresses with a 0 in the last octet valid? In my case, I have the the following netmask What about a 0 for the other octets?
Alan H's user avatar
  • 2,304
106 votes
15 answers

ssh tunnel refusing connections with "channel 2: open failed"

All of a sudden (read: without changing any parameters) my netbsd virtualmachine started acting oddly. The symptoms concern ssh tunneling. From my laptop I launch: $ ssh -L 7000:localhost:7000 user@...
lorenzog's user avatar
  • 2,879
99 votes
6 answers

SSHFS mount that survives disconnect

I'm using SSHFS mounts from my laptop to a central server. Obviously, the SSHFS mount is broken after a longer disconnect (eg. during suspend), cause the underlying SSH connection timed out. Is ...
bene's user avatar
  • 2,304
92 votes
7 answers

How do I make a connection private on Windows Server 2012 R2

After a restart of one of our servers (a Windows Server 2012 R2), all private connections become public and vice versa (this user had the same problem). Stuff like pinging and iSCSI stopped working, ...
vgru's user avatar
  • 1,297
91 votes
10 answers

Testing UDP port connectivity

I am trying to test whether I can get to a particular port on a remote server (both of which I have access to) through UDP. Both servers are internet facing. I am using netcat to have a certain port ...
Lock's user avatar
  • 1,657
91 votes
6 answers

How to determine the hostname from an IP address in a Windows network?

My LAN has 50 Windows hosts. At the Windows command line I try ping to get the IP address of a running Windows machine. The question is how to get hostname of a specific IP address in the same ...
billyduc's user avatar
  • 1,627
91 votes
8 answers

What is "anycast" and how is it helpful?

I'd never heard of anycast until a few seconds ago when I read "What are some cool or useful server/networking tricks?". The wikipedia "Anycast" article on it is quite formal and doesn't really evoke ...
username's user avatar
  • 4,755
90 votes
17 answers

Should network hardware be set to "autonegotiate" speeds or fixed speeds?

We recently had a little problem with networking where multiple servers would intermittently lose network connectivity in a fairly painful-to-resolve way (required hard reboot). This has been going on ...
Jeff Atwood's user avatar
  • 13.2k
87 votes
5 answers

How to get TX/RX bytes without ifconfig?

Since ifconfig is apparently being deprecated in major Linux distributions, I thought I'd learn something about the ip tool that's supposed to be used instead of ifconfig. And here I ran into a ...
justinas's user avatar
  • 1,008
87 votes
3 answers

What happens when you plug two sides of a cable to a single networking device?

What is likely to happen when you plug two ends of a network cable to a single switch/router? Will this create problems on the network, or just be ignored?
lisa1987's user avatar
  • 881
85 votes
8 answers

How can I list my open ports on Debian?

What is the command to display a list of open ports on a Debian server? I tried netstat -a | egrep 'Proto|LISTEN' but I would like something more specific that actually lists the port number.
leonel's user avatar
  • 989
85 votes
4 answers

How to reload default Mac OSX routing table without rebooting

Greetings, I'm using vpnc for a VPN client. I'm also doing some tricky things with route to make sure I can still access my local network, etc. etc. (the particulars here are not very important). ...
Nate Murray's user avatar
  • 1,003
82 votes
10 answers

How to check the physical status of an ethernet port in Linux?

I want to check if a specified ethX is physically up or down. How do I do that with the command line?
Jader Dias's user avatar
  • 4,735
79 votes
2 answers

what does "***" mean when traceroute

this is the result of my traceroute traceroute 1 ( 0.397 ms 0.380 ms 0.366 ms 2 ( 3.999 ms 3.971 ms 3.982 ms 3 (...
larry's user avatar
  • 4,067
78 votes
8 answers

How does Windows decide which DNS Server to use when resolving names?

What algorithm does Windows use to decide which DNS Server it will query in order to resolve names? Let's say I have several interfaces, all active, some with no dns server specified, some told to ...
Artefacto's user avatar
  • 1,065
76 votes
14 answers

Docker containers can't resolve DNS on Ubuntu 14.04 Desktop Host

I'm running into a problem with my Docker containers on Ubuntu 14.04 LTS. Docker worked fine for two days, and then suddenly I lost all network connectivity inside my containers. The error output ...
Thomas V.'s user avatar
  • 2,101
75 votes
6 answers

How to scan local network for SSH-able computers?

I am often on one computer in my house and I would like to SSH to another one, but often don't know the IP address of the one I want to connect to. Is there a way, from the command line, to scan the ...
Andrew's user avatar
  • 3,473
74 votes
4 answers

How to PREPEND rules rather than APPEND using iptables?

Pretty basic question: how to PREPEND rules on IPTABLES rather than to APPEND? I have DROP statements at the bottom of my rules. I have a software to add new rules but adding rules after DROP ...
ale's user avatar
  • 933
73 votes
6 answers

How do I delete a route from Linux routing table

This turns out to be harder than I thought. The routes I want to delete are the "!" rejected routes, but I can't seem to formulate the right "route del" command to pull it off. Here is the routing ...
user avatar
70 votes
4 answers

What is the "slash" after the IP? [duplicate]

In Amazon EC2, where I set "security groups", It says: Source: And then it gives an example of: What is "/24"? I know what port and IP is.
Alex's user avatar
  • 8,521
69 votes
8 answers

A previous IT worker probably left some backdoors. How can I eliminate them? [duplicate]

I started working for a company that fired a previous IT worker for leaking data. I can only say the following things: We use a Firebird DB with an application written by another company, Proxmox, ...
user2265690's user avatar
69 votes
6 answers

What is the difference between a managed and unmanaged switch?

I've seen some documentation discussing the use of an unmanaged switch. What is the difference in functionality/performance/etc. between an unmanaged and managed switch?
Luke's user avatar
  • 2,123
66 votes
19 answers

How do you document a network?

I'm not sure how to ask this question, since I'm not in the field. Say you're a network admin and you leave your job. How does the new guy know where to start?
Esteban Araya's user avatar
65 votes
6 answers

Why do ethernet cables have 8 wires?

This might seem a stupid question but why do Ethernet cables have 8 wires? Cat5 cables were just using 4 of the 8 wires, so only 4 are actualy 'needed'. Why not 12 or 16 wires?
ProgrammerAtWork's user avatar
64 votes
12 answers

Loopback to forwarded Public IP address from local network - Hairpin NAT

This is a Canonical Question about Hairpin NAT (Loopback NAT). The generic form of this question is: We have a network with clients, a server, and a NAT Router. There is port forwarding on the ...
adopilot's user avatar
  • 1,531
63 votes
5 answers

What is the difference between a public and private subnet in a Amazon VPC?

When I launch a server with a security group that allows all traffic into my private subnet, it displays a warning that it may be open to the world. If it is a private subnet, how can that be?
Developr's user avatar
  • 882
62 votes
7 answers

How to move files with SCP?

How to not copy but move files from one server to another (both Linux)? man scp didn't give me anything useful. I cannot use 'scp' then 'rm' because I must make sure the file is successfully ...
Putnik's user avatar
  • 2,277
60 votes
4 answers

How to manage DNS in NetworkManager via console (nmcli)?

I have CentOS 7.2 (guest in VirtualBox, vagrant box centos/7, no GUI). I see there is a nameserver in file: $ cat /etc/resolv.conf # Generated by NetworkManager nameserver But how to add or ...
Kirby's user avatar
  • 1,059
60 votes
2 answers

What is the ipv6 equivalent of

When describing IPv4 networks, I can use or just 0/0 to specify all networks. What is the equivalent notation for IPv6?
Andrew's user avatar
  • 1,144
60 votes
8 answers

What does "TTL expired in transit" mean on a ping attempt?

We get the message “TTL expired in transit” when we try to ping to a server in a different network segment. When we run tracert, 4 ip addresses repeat themselves indefinitely: 14 60 ms 59 ms ...
Leonardo's user avatar
  • 705
59 votes
1 answer

What does "Connection: close" mean when used in the response message?

When the client uses the Connection: close header in the request message, this means that it wants the server to close the connection after sending the response message. I thought that this header is ...
user365656's user avatar

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