TCP and IP are two important protocols used in networking today. Due to their importance, the Internet Protocol Suite is also commonly known as "TCP/IP"

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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 ...
49
votes
6answers
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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?
36
votes
11answers
29k 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 ...
36
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12answers
11k views

Why would you use IPv6 internally?

Of course, I realize the need to go to IPv6 out on the open Internet since we are running out of addresses, but I really don't understand why there is any need to use it on an internal network. I have ...
36
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6answers
215k views

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?
31
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8answers
11k views

When/why to start subnetting a network?

Under what conditions does one start to consider subnetting a network? I'm looking for a few general rules of thumb, or triggers based on measurable metrics that make subnetting something that should ...
30
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3answers
18k views

Tuning Linux IP routing parameters — secret_interval and tcp_mem

We had a little failover problem with one of our HAProxy VMs today. When we dug into it, we found this: Jan 26 07:41:45 haproxy2 kernel: [226818.070059] __ratelimit: 10 callbacks suppressed Jan 26 ...
25
votes
13answers
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How to get a list of all IP addresses (and ideally device names) on a LAN?

Our network gave an error that there was an IP address conflict and I'd like to find what all the device IP addresses are. (I've also had need of that before). (update/clarification:I'm looking for a ...
24
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16answers
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192.168.1.x more exploitable?

Our IT services firm is proposing a network reconfiguration to use the IP range 10.10.150.1 – 10.10.150.254 internally as they state the current IP scheme using manufacturer defaults of 192.168.1.x is ...
21
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2answers
29k views

What is the maximum port number?

I'd like to set some linux services to non-standard ports - what's the highest valid port number?
20
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2answers
9k views

Unix socket vs TCP/IP host:port

Could someone please describe to me the pros and cons of using a Unix socket file vs a tcp/ip localhost:port when setting up services on a server (Ubuntu, FWIW)? In this particular instance it's for ...
19
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9answers
14k views

What is the OSI model and how does it apply to todays networks?

As a system administrator what should I know about the OSI model? (Rec X.200) Briefly, what are the layers and what do they do? How much do I need to know about in my day-to-day work? How does this ...
14
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5answers
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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 ...
14
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4answers
55k views

Which is the default TCP connect timeout in Windows?

Which is the default TCP connect timeout in Windows? There is a registry key to configure it or it's set dynamically?
14
votes
1answer
3k views

What am I misunderstanding about calculating MTU?

Okay, I just finished resolving a jumbo frames issue between a few Xserves, a Netgear GSM7224, and a Drobo B800i. It turned out that the Xserves (Mac OS X 10.6.8 Server) and the Drobo B800i accept the ...
13
votes
4answers
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What does a DNS request look like?

I have seen what the text representation of an HTTP request is, but what does a DNS request look like? Where in the data is the location of the URL you are trying to locate? Also, how is the ...
13
votes
3answers
3k views

How are IP addresses actually assigned?

I'm having a hard time understanding how a governing body assigns IP addresses, companies use BGP to advertise those IPs, and how the internet works. Then, where the hell does DNS come in? Can ...
12
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3answers
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How does the HTTP GET method work in relation to DNS protocol?

I am trying to understand application layer protocols in TCP/IP stack. I know that both HTTP and DNS protocol stay at the top layer (Application Layer). So, when a browser wants to access a resource, ...
12
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6answers
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When IP aliasing how does the OS determine which IP address will be used as source for outbound TCP/IP connections?

I have a server running Ubuntu Server with four IP addresses aliased on a single NIC. eth0 192.168.1.100 eth0:0 192.168.1.101 eth0:1 192.168.1.102 eth0:2 192.168.1.103 (Using ...
12
votes
3answers
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Slash Notation IP - What is what?

We just signed up with a new ISP and we got a static IP from them. Our previous ISP just gave one IP and we were able to configure our web server using that. Now, we have got this new IP with a slash ...
11
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2answers
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Multiple IP addresses per NIC

How can I have multiple IP addresses assigned to a single NIC? I remember doing this on Unix way back when. Can it be done on Windows?
9
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3answers
5k views

Dead gateway detection on Windows 2008 Server

We have recently implemented HAProxy for stackoverflow.com. We decided on using TProxy to maintain the source address for clients connecting so our logs and other IIS modules which depend on the ...
9
votes
7answers
10k views

Tuning NFS client/server stack

I have a CentOS 5 VMWare server connecting to an OpenSolaris 2009.06 machine over NFS that holds the disk images. My virtual machines seem to be bound by slow IO so I'd like to do everything I can to ...
8
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6answers
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How do I view the TCP Send and Receive Queue sizes on Windows?

Linux netstat shows send and receive queue sizes. How do I get this info under Windows, specifically Server 2003?
8
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3answers
2k views

Can I make TCP/IP session to run less than 60 seconds?

Our server is overloaded with TCP/IP sessions, we have 1200 - 1500 of them. Most of them are hanging in TIME_OUT state. It turns out that a connection in TIME_OUT state occupies a socket until 60 ...
8
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1answer
9k views

What is the maximum number of TCP connections I can have in Windows Server 2008?

I would like to have as many connections (single connections from many different clients) as humanly possible in a server running on Windows Server 2008, in order to support a Comet-style application. ...
7
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2answers
2k views

TCP Dump, cannot understand these 4 lines?

I need support understanding these 4 lines. looks like tcp dump but im actully not understanding what exactly is happening here. 13:13:22.407445 IP 192.168.246.128.54955 > 192.168.246.13.80: S ...
7
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1answer
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limits.conf setting doesn't 'stick'

Trying to get the equivalent of ulimit -n 20000 on a high load web server, however the /etc/security/limits.conf on ubuntu hardy doesn't seem to stick. here is what I set in the config file. root ...
7
votes
4answers
5k views

How to automatically assign a route to client in Windows Server PPTP VPN?

Is there a way to configure PPTP VPN on Windows Server which automatically tells the client to add a route for the private network? I have to do it manually on every PC (once) using following command ...
7
votes
1answer
1k views

TCP/IP performance tuning under KVM/Qemu

With more and more companies switching to public cloud services, I'm curious what you guys' thoughts are on TCP/IP tuning in the cloud. Is it worth bothering with? Given that you don't have access to ...
6
votes
8answers
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How to find the gateway IP address in Linux

What command can you use to find the Gateway IP Address (ie. home router address) for eth0 in Linux? I need to get the IP address from a command line app to use in a shell script.
6
votes
5answers
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Tunnel over UDP or TCP

Is there a way to create something similar to an IPIP or GRE tunnel only over UDP or TCP? I have a GRE tunnel between two servers and I noticed that sometimes it starts to drop packets. At the same ...
6
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3answers
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Where is the documentation that states TCP and UDP source port should be over 1024 and random?

I am having a hard time to find where it is documented that the source port should be random and in the range 1024-65535. In which RFC is this documented? Edit: First reference for privileged ports ...
6
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1answer
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HTTP, TCP, UDP and connectionless

I am a bit confused with HTTP lately. Some facts are that TCP can operate connection orientated or connectionless, this I understand. TCP though is connection-oriented while UDP is connectionless ...
6
votes
4answers
14k views

Running lsof -i shows a lot of connections in CLOSE_WAIT ? Should I worry

So I am running lsof -i | wc -l periodically and it is telling me that out of 420 lines, between 240 and 255 are in CLOSE_WAIT state. How does TCP connections enter this state ? Should I be worried ...
6
votes
1answer
1k views

What are some TCP tuning tips for a service hit by iPhone clients on mobile networks like 3G?

I am a developer that has the good+bad situation of designing a network service that will be hit very hard by iPhone clients. The iPhone app has over 10MM downloads in the past year and now I'm ...
6
votes
2answers
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Windows 7 TCP Listen Ports

My fresh install of Windows 7 shows 5 programs listening on TCP ephermeral ports: TCP 0.0.0.0:49152 0.0.0.0:0 LISTENING TCP 0.0.0.0:49153 0.0.0.0:0 ...
5
votes
7answers
946 views

Unencrypted equivalent of SSH reverse proxy

I'm looking for an equivalent of this: ssh -R 8888:10.0.0.2:8888 100.101.102.103 ie. make the service running on port 8888 on the local machine (10.0.0.2) look like a locally running service on a ...
5
votes
7answers
8k views

Latency in TCP/IP-over-Ethernet networks

What resources (books, Web pages etc) would you recommend that: explain the causes of latency in TCP/IP-over-Ethernet networks; mention tools for looking out for things that cause latency (e.g. ...
5
votes
6answers
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Looking for an open source real-time network analysis program

Can somebody recommend an open source real-time network analysis program? What I'm looking for the program to do is display a graph of bandwidth usage by IP within our internal network that can ...
5
votes
2answers
12k views

What do all the settings in the Advanced TCP/IP Properties DNS Tab Mean?

Can someone explain TCT/IP > Properties > Advanced > 'DNS' tab please? I know what DNS, primary DNS suffix and DNS suffix are, but this tab has some confusing properties.
5
votes
1answer
795 views

What does a magic packet look like?

What does a magic packet contain and what options does the protocol provide?
5
votes
2answers
4k views

How do I set up IP over FC?

I have 2 systems, with a Broadcom BCM5701 and a LSI7202p, of which are connected with a LC->SC cable. The cards are properly detected in Linux, however I wish to set up IP over FC. I've read online ...
5
votes
10answers
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Forward port to another Ip/port

I recently found out that my ISP disables port 135. I have a server and personal computer behind my router. A friend and I are working on a programming project and are running into some sql issues ...
5
votes
2answers
3k views

Answering on the same interface where the request came from

First, I know this is not how IP is intended. But as usual, this is how I need to have it working. I work for a security company. There are at least 2 network interfaces. Both interfaces should have ...
5
votes
2answers
3k views

Unable to increase the dynamic port range on Windows Server 2008

I'm trying to "tune" the TCP/IP stack on some Windows Server 2008 machines by following the instructions here: ...
5
votes
1answer
11k views

How do I setup a TCP over DNS tunnel on Windows?

I've seen a few guides out there, but I could not get any of them to work. I can point NS records to my domain, so it sounds like I should be able to do this. I tried setting up a DMZ for the server ...
5
votes
1answer
144 views

Sender Information Not Cached on the Receiver During an ARP Request

I'm currently trying to get my head around more advanced networking concepts. The way I am doing this is by going on one of the servers I look after as a sysadmin and running network captures to see ...
5
votes
1answer
882 views

Where are my TIME_WAITs on Mac OS X?

No TIME_WAITs on Mac OS X Normally, when a TCP connection is closed, the socket on the side where close() is called first is left in the TIME_WAIT state. When one of the peers is a Mac OS X (Lion) ...
4
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3answers
2k views

Are there simpler ways to do traffic shaping in linux besides tc?

I read lartc but I don't feel I need such a complicated solution for simple needs. One of such simple use cases is to limit a specific process' available bandwidth, upstream or downstream. How can I ...