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

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288
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6answers
76k views

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 ...
19
votes
7answers
14k views

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 ...
93
votes
7answers
98k views

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 ...
68
votes
2answers
53k views

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 ...
48
votes
7answers
33k views

What is “anycast” and how is it helpful?

I'd never heard of anycast until a few seconds ago when I read this SF post. The wikipedia entry on it is quite formal and doesn't really evoke a mental picture of how it would be used. Can someone ...
48
votes
3answers
2k views

Assessing equipment damage following a lightning strike - Should I have planned more?

One of my client's sites received a direct lightning hit last week (coincidentally on Friday the 13th!). I was remote to the site, but working with someone onsite, I discovered a strange pattern of ...
61
votes
17answers
26k views

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 ...
102
votes
4answers
19k views

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 ...
24
votes
6answers
14k views

CIDR for Dummies

I understand what CIDR is, and what it is used for, but I still can't figure out how to calculate it in my head. Can someone give a "for dummies" type explanation with examples?
65
votes
3answers
26k views

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 ...
17
votes
7answers
25k views

Force local IP traffic to an external interface

I have a machine with several interfaces that I can configure as I want, for instance: eth1: 192.168.1.1 eth2: 192.168.2.2 I would like to forward all the traffic sent to one of these local ...
52
votes
6answers
42k views

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

Are IP addresses with a 0 in the last octet valid? 10.6.43.0 In my case, I have the the following netmask 255.255.252.0 What about a 0 for the other octets?
3
votes
4answers
786 views

ssh delay when connecting

When connecting to one specific server (running Debian Lenny) it always takes about 5 seconds before it prompts me to enter a password. After login there is no noticable delay anymore. There is also ...
60
votes
11answers
131k views

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, ...
15
votes
5answers
71k views

Huge amount of TIME_WAIT connections says netstat

Okay, this is creeping me out - I see about 1500-2500 of these: root@wherever:# netstat Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 ...
51
votes
12answers
5k views

What should I pay attention to when I'm buying a network switch?

Since I'm not a hardware expert, I don't know what features make a network switch a good network switch. What should I pay attention, when I'm comparing the different models from different vendors?
9
votes
2answers
19k views

Load balancing & NAT-ing multiple ISP connections on Linux

I have two internet connections from two different ISPs and I need to balance the traffic originating to and from my network between the two ISPs. I am using Debian GNU/Linux. My set-up is like this ...
44
votes
7answers
7k views

Probability of Blade Chassis Failure

In my organisation we are thinking about buying blade servers - instead of rack servers. Of course technology vendors also make them sound very nice. A concern, that I read very often in different ...
14
votes
4answers
4k views

Managing an application across multiple servers, or PXE vs cfEngine/Chef/Puppet

We have an application that is running on a few (5 or so and will grow) boxes. The hardware is identical in all the machines, and ideally the software would be as well. I have been managing them by ...
7
votes
1answer
13k views

How to troubleshoot latency between 2 linux hosts

The latency between 2 linux hosts is about .23ms. They are connected by one switch. Ping & Wireshark confirm the latency number. But, i dont have any visibility into what is causing this latency. ...
55
votes
20answers
18k views

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?
17
votes
7answers
13k views

How many VLANs are too few and too many?

We're currently running a netwok of 800+ PCs and 20+ servers, the network infrastucture is along the lines of Core Switch 10Gb-> Area Switch 2GB-> Local Switch 1GB-> Desktop. All running 3Com ...
4
votes
5answers
28k views

Windows networking performance (SMB/CIFS)

Are there registry settings or other settings available to tune Windows networking (SMB/CIFS) performance? I'm trying to get maximum throughput for large file copy operations but any settings would ...
19
votes
6answers
33k views

What are the implications of having two subnets on the same switch?

Can anyone tell me what some of the implications of having two different subnets on the same switch would be if VLANs are not being used?
3
votes
4answers
3k views

What is the correct behaviour when Nameservers Change

I have been having some issues with a certain network supplier over unchanged nameservers making my site invisible to their customers. I wouldnt be bothered except that they are one of the largest ...
8
votes
4answers
2k views

Multi-WAN bonding across different media

I've recently been thinking again about a product that Viprinet provide, basically they've got a pair of routers, one that lives in a datacentre, Their VPN Multichannel Hub and the on-site hardware, ...
2
votes
3answers
828 views

Can I have web server on a home network with 1 public IP address? [closed]

I've got a fairly standard home network with a modem from my ISP, which is connected to a router which then provides NAT to about 3 PCs and laptops. There's a possibility to obtain one public IP from ...
74
votes
7answers
102k views

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 ...
12
votes
3answers
6k views

What is needed to use anycast IPs?

So, there're a bunch of questions on SF about the uses and how anycast IPs are cool. My approach is something more practical. What specifically I need to have to use one of those addresses? Do I ...
17
votes
2answers
1k views

Are there any advantages to using someone else's publicly-routable IPv4 addresses locally?

My 10000+ users network, which spans the whole State and is very complex, has a "strange" addressing scheme. Though our PCs are not directly connected/exposed to the Internet, our network designers ...
3
votes
1answer
1k views

How to add a broadcast address to loopback with ifconfig on a OS/X?

I am trying to use ifconfig to turn on broadcast on my loopback interface. It currently reads: lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384 As you can see, no broadcast address! ...
0
votes
2answers
109 views

copying a file 2 connections away?

I dislike calling scp twice. I need to connect to box1 via ssh the again to boxNAME. How do i copy a file from boxNAME to my local drive? also how do i copy a file from my local drive back to ...
224
votes
15answers
14k views

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 ...
74
votes
8answers
62k views

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 ...
161
votes
13answers
27k views

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), ...
12
votes
8answers
16k views

Troubleshooting a “slow” network

We've all had a complaint that the "network" is "slow" at some point: might be localized to one room (switch) or one computer, might just be Internet (DNS? Browser issue?), might be just one ...
36
votes
11answers
30k views

What is the difference between UDP and TCP?

My router has two protocols (and a "both" option) that I can select when setting up port forwarding: UDP and TCP. What is the difference between these two protocols and when would you select one over ...
14
votes
5answers
85k views

How to route different traffic thru different network interfaces (in Windows)

I've searched for details on how to do this but I've been unsuccessful - I wondered if someone could offer up some advice. So, let's say I have 2 network cards (LAN and 3G in my instance), both ...
11
votes
6answers
50k views

Windows7 - “The specified network password is not correct.” when the password is in fact correct

I have a samba server setup for some time now. It is a Hardware NAS - which unfortunately does not provide access to the Samba logs. (the exact model of the NAS is called Addonics NAS Adapter ) I ...
41
votes
8answers
315k views

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 ...
28
votes
6answers
5k views

Hardware Firewall Vs. Software Firewall (IP Tables, RHEL)

TL;DR My hosting company says IP Tables is useless and doesn't provide any protection. Is this BS? I have two, co-located servers. Yesterday my DC company contacted me to tell me that because I'm ...
7
votes
2answers
14k views

Bridging LXC containers to host eth0 so they can have a public IP

UPDATE: I found the solution there: http://www.linuxfoundation.org/collaborate/workgroups/networking/bridge#No_traffic_gets_trough_.28except_ARP_and_STP.29 # cd /proc/sys/net/bridge # ls ...
18
votes
7answers
3k views

Why don't more organizations use inside-to-inside NAT or similar solutions to allow NAT hairpins?

Inside-to-inside NAT aka NAT loopback solves hairpin NAT issues when accessing a web server on the external interface of an ASA or similar device from computers on the internal interface. This ...
10
votes
2answers
3k views

linux command to prevent dos attack by using netstat and iptables

I want to DROP more than 200 requests per ip to prevent ddos attack. this is command that i used to detect requests count per ip : netstat -alpn | grep :80 | awk '{print $5}' |awk -F: '{print ...
18
votes
8answers
14k views

What is the network address (x.x.x.0) used for? [duplicate]

It appears to be common practice to not use the first address in a subnet, that is the IP 192.168.0.0/24, or a more exotic example would be 172.20.20.64/29. The ipcalc tool I frequently use follows ...
12
votes
1answer
8k views

Using dnsmasq to resolve all hosts to the same address

I'm running Ubuntu server on a computer used as a wireless AP, but this AP should resolve all DNS requests to an internal IP address rather than actually performing the lookup. I want to do the same ...
11
votes
6answers
16k views

How does geography affect network latency?

I have the option of hosting our database/web server at a managed hosting company on the East Coast (US) or one on the West Coast (US). Our company is based out of New York City, and both hosting ...
6
votes
2answers
3k views

Debian 'ignores' /etc/network/if-pre-up.d/iptables

I want my iptables rules to automatically be loaded on startup. According to the wiki on Debian this can be done by placing a script with the name iptables in /etc/network/if-pre-up.d/ So I did, this ...
4
votes
6answers
4k views

Wireless to Wireless Transfer Slow on a Linksys WRT54GL

The Situation: When I try to transfer a file from one computer to another that are both connected via wireless on a WRT54GL (in a office) with dd-wrt firmware I often get bad speeds. In generally ...
2
votes
2answers
147 views

Disable IP on bridge

OK, I'm running libvirt on my server, I have only one interface. In order to use the public IP in my VM I need to set up a bridge, everything OK so far.. But I don't want to setup an IP address on the ...