ITS CSS Desktop Support Introduction to networking concepts Last updated: 9/30/2008 by pxahelp

Internet Protocol IP is used for communicating data across a packet-switched network. Nodes within a network are assigned an IP address. IP acts as a transport layer for other network protocols (ICMP, TCP, UDP)

MAC Addresses A MAC Address is a universally unique identifier (UUID) attached to a given Network interface (NIC). A MAC Address consists of 6 groups of 2 hexadecimal digits, ex: 01:23:45:67:89:af DHCP servers often assign IP Addresses based on MAC.

IP Addresses An IP consists of 4 “octets” Each octet is a binary byte (8-bits), commonly represented in decimal = = = 255 The IP address is split into a network address and host address using a “subnet mask”

Domain Names A domain name like claws.rit.edu corresponds to a single IP Address. RIT’s DNS servers: DNS servers are provided by DHCP, and use a simple “query-response” protocol to resolve DNS names to IP Addresses.

From Domain Name to MAC 1. DNS query: claws.rit.edu 2. DNS response: ARP query: Who has ? Tell ARP response: 00:19:B9:21:06:DD This is overly simplified, but the actual discussion is out of scope for this presentation.

IP Subnetting Originally the first octet was used to denote the network and the remainder were the node. Obviously this is very inflexible. RIT’s network address is Subnetting was devised as a way for an organization which controls its own DHCP and DNS to further divide its address space.

IP Subnetting A subnet mask uses the same format as an IP address, where each bit who’s value is 1 is part of the network and everything else identifies the computer. The RIT subnet mask is:

IP Subnetting RIT itself is a “subnet” of the rest of the internet, however its subnet mask is The remaining in binary is 9 bits of 1’s and 7 bits of 0’s. So we say that RIT uses a 9-bit subnet.

9-bit subnetting What does this mean? Using 9 bits gives us 512 possible subnets. (counting 0 and 255) This is 256 (0-255) values for the 3 rd octet and 256 values for the 4 th octet. Since only a single bit of the subnet mask applies to the 4 th octet only 2 of those 256 values refer to subnets. At RIT we refer to this division as “high” and “low” nets. (ex: 233 high means an IP address above )

9-bit subnetting An IP address whose node address is empty (all 0s in binary) is considered to be the subnet ID An IP address whose node address is full (all 1s in binary) is considered to be the broadcast address for that subnet.

Meaning? To target a network (for registration, for example) you would use Low: *.*.*.0 High: *.*.*.128 To send something to every machine on that net (broadcast ping for switch testing) you would use Low: *.*.*.127 High: *.*.*.255

Subnet address allocation In Low Subnets Addresses are available for permanent registrations Addresses are reserved for roaming workstations and are dynamically allocated by DHCP Addresses are reserved for networking hardware (Fluke, routers, firewalls)

Subnet address allocation In High Subnets Addresses are available for permanent registrations Addresses are reserved for roaming workstations and are dynamically allocated by DHCP Addresses are reserved for networking hardware (Fluke, routers, firewalls)

Edge cases In rare instances you will see an 8 bit subnet. This is distinguishable from a standard 9 bit net by its subnet mask. 8-bit subnet mask: Using 8 bits instead of 9 creates fewer subnets with more available host addresses. An 8-bit net does not have a high and low. Its subnet ID is *.*.*.0, like a “low” subnet. However the broadcast address for one is *.*.*.255, like a “high” subnet.

Diagnostic Tools: Windows ipconfig /all – ipconfig will display configuration information for the computer. ipconfig will display some information adding the /all will display more information. ipconfig /release – this will “release” the IP information so the computer can obtain new information ipconfig /renew – this will get the new information from the network for the computer ping – Sends packets to a specified node to test the connectivity between two nodes. tracert – similar to ping, this will send packets to a node on the network but it reports back information on each node it passes by to get the node requested. nslookup – uses DNS to translate an IP into a hostname or a hostname into an IP.

Diagnostic Tools: Linux/Mac dig – An interface to the DNS servers, similar to nslookup, but with far more flexibility and very clear output. Not always available. ifconfig – InterFace Config. The UNIX version of ipconfig. Displays broadcast address, subnet mask, current IP, and MAC on a per NIC basis. ifconfig eth0 down, ifconfig eth0 up is equivilant to ipconfig /release and ipconfig /renew Mac does not use eth0 names, use en0 on Mac. dhclient – Common Linux command (Gentoo, SuSE, Red Hat) to renew the IP address without disabling the adapter. nslookup – The same as the Windows version of this command. Runs a simple query against a DNS server and displays the results. ping – Mostly the same as the Windows version of this command. It is important to know that UNIX ping will not stop after 4 pings unless told. Ctrl+c will quit a running ping. traceroute – Behaves the same as Windows tracert