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468

IP subnets exist to allow routers to choose appropriate destinations for packets. You can use IP subnets to break up larger networks for logical reasons (firewalling, etc), or physical need (smaller broadcast domains, etc). Simply put, though, IP routers use your IP subnets to make routing decisions. Understand how those decisions work, and you can ...


92

It depends on the subnet of the IP address in question. In general, the first and last addresses in a subnet are used as the network identifier and broadcast address, respectively. All other addresses in the subnet can be assigned to hosts on that subnet. For example, IP addresses of networks with subnet masks of at least 24 bits ending in .0 or .255 can ...


83

Continued from: How does IPv4 Subnetting Work? Your ISP gives you the range the network ID 192.168.40.0/24 (11000000.10101000.00101000.00000000). You know that you'd like to use a firewall / router device to limit communication between different parts of your network (servers, client computers, network equipment) and, as such,you'd like to break these ...


62

You can find a good summary here: What is the difference between UDP and TCP internet protocols? Both TCP and UDP work at transport layer TCP/IP model, but have very different usage. The most important differences are: Reliability: TCP: connection-oriented UDP: connectionless Ordered: TCP: order of message receipt is guaranteed UDP: order is not ...


58

Sub-netting Sub-netting is not difficult but it can be intimidating. So let's start with the simplest possible step. Learning to count in binary. Binary Binary is a base 2 counting system. Consisting of only two numbers (1 and 0). Counting proceeds in this manner. 1 = 001 ( 0 + 0 + 1 = 1) 2 = 010 ( 0 + 2 + 0 = 2) 3 = 011 ( 0 + 2 + 1 = 3) 4 = 100 ( 4 + 0 ...


56

This will add at best a very thin layer of "security by obscurity", as 192.168.x.y is a way more commonly used network address for private networks, but in order to use the internal addresses, bad boys have to be already inside your network, and only the most stupid attack tools will be fooled by the "non standard" address scheme. It cost nearly nothing ...


42

There is no NAT for IPv6 (as you think of NAT anyway). NAT was an $EXPLICATIVE temporary solution to IPv4 running out of addresses (a problem which didn't actually exist, and was solved before NAT was ever necessary, but history is 20/20). It adds nothing but complexity and would do little except cause headaches in IPv6 (we have so many IPv6 Address we ...


38

And the CEO level explanation: UDP is when you throw your paper in the general direction of the bin. TCP is when it misses, you throw exact copies of the same paper again and again until it falls into the bin. There would be paper wastage, even resent TCP packets result in wastage of network or system resources.


30

Sounds like billable busywork to me. Aside from the fact that many consumer appliances use the 192.168.x.x address space (which can be exploited, like anything else), I don't feel that really changes the security landscape of a corporate network. Things inside are locked down, or they aren't. Keep your machines/devices on current software/firmware, follow ...


30

I found a hiddem gem the other day from Microsoft that is designed for testing ports: Portqry.exe "Portqry.exe is a command-line utility that you can use to help troubleshoot TCP/IP connectivity issues. Portqry.exe runs on Windows 2000-based computers, on Windows XP-based computers, and on Windows Server 2003-based computers. The utility reports the port ...


28

Using nmap to do a sweep of the subnet is one quick and simple way to do this that I've used before, the various options will allow you to do a more detailed inspection also.


28

HTTP request in question is actually not valid unless browser is talking to intermediary (proxy). Your example would look bit more like following if browser was talking with web server directly: GET /hello.htm HTTP/1.1 Host: www.pippo.it Now to put it in perspective consider OSI layer model So we have 3 systems in action. Client running the browser ...


26

Interesting Question. Historically, prior to the advent of fully switched networks, the main consideration to breaking a network into subnets had to do with limiting the number of nodes in a single collision domain. That is, if you had too many nodes, your network performance would reach a peak and eventually would collapse under heavy load due to excessive ...


26

I never ever encountered this issue. However, you should probably increase your hash table width in order to reduce its depth. Using "dmesg", you'll see how many entries you currently have: $ dmesg | grep '^IP route' IP route cache hash table entries: 32768 (order: 5, 131072 bytes) You can change this value with the kernel boot command line parameter ...


24

(2^16)-1, or 0-65,535 (the -1 is because port 0 is reserved and unavailable). (edited because o_O Tync reminded me that we can't use port 0, and Steve Folly reminded me that you asked for the highest port, not the number of ports) But you're probably going about this the wrong way. There are people who argue for and against non-standard ports. I say ...


24

Yes, it can be done in Windows: Go to the Control Panel > Network Connections Right click on the Local Area Connection (or whichever network connection you want to add the 2nd IP Address) and click Properties Click on Internet Protocol (TCP/IP) in the connection box and click properties Enter the first IP address in the properties box Click Advanced Click ...


24

If there's a unix box on the network, you could try arp-scan: http://linux.die.net/man/1/arp-scan $ arp-scan --interface=eth0 192.168.0.0/24 Interface: eth0, datalink type: EN10MB (Ethernet) Starting arp-scan 1.4 with 256 hosts (http://www.nta-monitor.com/tools/arp-scan/) 192.168.0.1 00:c0:9f:09:b8:db QUANTA COMPUTER, INC. 192.168.0.3 ...


23

This is a raw dump from Wireshark of a DNS query. The DNS part starts with 24 1a: 0000 00 00 00 00 00 00 00 00 00 00 00 00 08 00 45 00 ........ ......E. 0010 00 3c 51 e3 40 00 40 11 ea cb 7f 00 00 01 7f 00 .<Q.@.@. ........ 0020 00 01 ec ed 00 35 00 28 fe 3b 24 1a 01 00 00 01 .....5.( .;$..... 0030 00 00 00 00 00 00 03 77 77 77 06 67 6f 6f ...


21

A brief history lesson: originally, unicast IPv4 addresses were divided into 3 classes, each with an associated 'default' mask length (called the classful subnet mask) Class A: Anything in the range 1.0.0.0 -> 127.255.255.255. Classful subnet mask of 255.0.0.0 (/8 in CIDR notation) Class B: Anything in the range 128.0.0.0 -> 191.255.255.255. Classful ...


19

Unix sockets are a little bit faster as you don't have the tcp-overhead. If you realize this performance loss is a question of server load. If you don't have very high server load you won't recognize it. If you use Jails (FreeBSD) or some other virtualisation technology to separate the e.g. MySQL-Server from the Webserver, you often use the tcp/ip setup ...


18

Running out of internal (rfc1918) ipv4 addresses can also be a very valid reason to go ipv6. Comcast explained at Nanog37 why they were going ipv6 for their management addresses. 20 Million video customer x 2.5 STB/customer x 2 ip addresses/STB -------------------- = 100 Millions IP addresses And this is only for video, not data/modems. They ...


17

Leased IP Blocks IPs are assigned in blocks by IANA to the Regional Internet Registries (RIR). See this (list and map) of the RIRs. The RIRs then lease out smaller blocks IPs to individual companies (usually ISPs). There are requirements (including fees and proof of use) for getting a distribution and failing to maintain these means a loss of lease. Once a ...


16

Seems like client 192.168.246.128 tried to connect to web server 192.168.246.13 but client's window size of 92 bytes was refused by a slow-read attack prevention mechanism.


15

I don't see anything wrong with being able to differentiate layers between the two models. On the plus side, having knowledge of the OSI model comes in handy when you're talking to people or reading documentation referencing the top 4 layers of the OSI model: Here are some links with explanation on what each layer does: ...


14

1-65535 are available, and ports in range 1-1023 are the privileged ones: an application needs to be run as root in order to listen to these ports.


13

The /30 means all but two of the 32 bits are used to define the netmask. It also means you've got four IP addresses to play with. (But in reality, only two of which can be hosts) Your last octet of the netmask is (in binary) 11111100, which leaves the last two bits for you to define your network. (hence the four addresses) Just looking at the last two ...


13

This is going to be slightly tricky if you're intending to treati this as one big subnet, because 192.168.1.0 to 192.168.2.255 isn't correctly aligned on the right boundary for a /23 subnet, so you can't treat it as 192.168.1.0/23. If you're completely set on using specifically 192.168.1 and 192.168.2 then you'd need to use a subnet of 192.168.0/22, which ...


13

Give openvpn a shot. You can create tunnels over UDP or TCP.


12

Use the telnet command to connect to the server on the specified port, and see if a connection can be established. Success: $ telnet my_server 25 220 my_server ESMTP Postfix Fail: $ telnet my_server 23632 Connecting To my_server...Could not open connection to the host, on port 23632: Connect failed



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