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416

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 ...


81

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 ...


58

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 ...


55

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 ...


48

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 ...


39

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 ...


28

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 ...


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 ...


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 ...


23

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 ...


21

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 ...


21

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 ...


18

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 ...


17

(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 ...


17

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 ...


15

The less-quick-than-it-used-to-be version: IPs are assigned in blocks by IANA to the Regional Internet Registries (RIR). See this (list and map) of the RIRs for more. The RIRs then lease out the IPs to individual companies (usually ISPs). There are requirements (including monetary and proof of use) for getting a distribution and failing to maintain these ...


15

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 ...


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: ...


12

ifconfig is usually in /sbin. Depending on what system you're running this mightn't be on your path (in particular, RedHat systems don't include this on the default path) Try: $ /sbin/ifconfig If it works then that's the issue, and you should change your path via your .bashrc (or whatever shell you're using). export PATH=$PATH:/sbin:/usr/sbin


12

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 ...


12

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 ...


12

You are probably looking for RFC 6056 - Recommendations for Transport-Protocol Port Randomization ("Best Current Practice"). Technically there is no requirement that the ephemeral port be >1024 or random (you could build a system that always initiates connections from port 12 because you like the number 12), it's just not "normal" to do so (and an awful ...


10

My old job, at a large University, would use an IPv6 allocation internally. They were assigned an IPv4 /16 back in the day and even today is passing out IPv4 addresses to nearly every internal client. The RFC1918 networks were restricted to the telecom-only network and certain specialized usages (the PCI standards required RFC1918 usage until October 2010). ...


10

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 ...


10

From the Wikipedia Entry on Wake on LAN The Magic Packet is a broadcast frame containing anywhere within its payload 6 bytes of ones (resulting in hexadecimal FF FF FF FF FF FF) followed by sixteen repetitions of the target computer's MAC address.


10

You can use a thing called a Static Route to indicate which connection you wish to be used (if they are both on the same subnet), or you can manually modify your HOSTS file (if they're on a different subnet). For starters, it's generally a good idea to have the crossover connection (FYI Gigabit does not require a crossover cable, it will automatically ...


10

As mentioned by others, having some connections in TIME_WAIT is a normal part of the TCP connection. You can see the interval by examining /proc/sys/net/ipv4/tcp_fin_timeout: [root@host ~]# cat /proc/sys/net/ipv4/tcp_fin_timeout 60 And change it by modifying that value: [root@dev admin]# echo 30 > /proc/sys/net/ipv4/tcp_fin_timeout Or permanently ...


10

Welcome to the strange and mysterious world of jumbo frames! It's normal that jumbo frame ethernet gear has MTU > 1518 and < 65K byte and you have to find a setting that is the lowest common denominator across your L2 domain to enable proper jumbo traffic. My guess is that your ping/ICMP implementation only works for 8192 bytes payload, so 8164 + 28 ...


9

HTTP (1.0 without connection keep alive) is connectionless because once a single HTTP request is serviced, the connection is closed and not reused. HTTP requests are not TCP protocol data units, so that TCP is connection-oriented with respect to TCP protocol data units doesn't stop HTTP from being connectionless with respect to HTTP protocol data units. ...



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