Network Addresses, Switches and Routing Bob Bradley IS361 Fall 2005 Chapter 1 Part 2

ipconfig /all

Description : Broadcom 54g MaxP Physical Address : B C8 Dhcp Enabled : Yes Autoconfiguration Enabled.... : Yes IP Address : Subnet Mask : Default Gateway : DHCP Server : DNS Servers : Lease Obtained : Sunday, September Lease Expires : Sunday, September

Cable Modem The cable modem hooks up using the same coax cable as your TV. The cable modem has a RJ-45 jack on it. You plug your PC into this jack using a standard UTP (Unshielded Twisted Pair) Ethernet cord. The cable company typically dynamically assigns one IP address to the cable modem. When the cable modem starts up, it automatically retrieves the assigned IP address (and other configuration information) and passes along to the PC. Cable Modem Coax to Cable Company UTP PC with Ethernet NICs

Cable/DSL Access Routers Allow you to share broadband connections with more than one PC Example: –LinkSys Broadband Routers –D-Link –Netgear –Blitzz

Sharing a Cable Modem with an Access Router Access Router DHCP Server NAT Server b Wireless AP Ethernet Switch Router / Firewall Notebook with b wireless card Wireless LAN Cable Modem Coax to Cable Company UTP WAN Port PC with Ethernet NICs LAN UTP Printer Server Printer with Ethernet port

Ethernet Switch The switch passes messages (frames) between locally attached devices –PCs (desktops & notebooks), Network Printers –X-Box / Playstation –P-DVRs / Tivos, Network Storage, Internet Radios / MP3 Players The switch in the access router also passes messages to the router module and other modules: –Router, NAT Server, DHCP Server, Wireless AP, Printer Server

Switches & Ethernet Addresses Switches only look at the Ethenet (LAN / MAC) addresses –Switches do not know or understand IP addresses They do not know or understand anything about the Internet Switches learn the Ethernet address of each device attached to the LAN and keep track of which port the device is plugged into –When a frame comes in on a port, the switch looks at the from address and adds the port/address pair to a list Therefore when a frame comes in, a switch can send that frame back out only the port that it needs to go to

Ethernet Addresses Ethernet addresses are assigned at the factory to each Ethernet card –They are “burned” into the card’s ROM –They are usually displayed as HEX numbers –Example: B C8 –They are assigned in batches to companies You can look up the manufacture of a card by looking at the first few numbers of the Ethernet address

Router / Firewall Passes IP messages (packets) –From the WAN (Internet) to the LAN –From the LAN to the WAN (Internet) Usually has a built in firewall / packet filter –This prevents unsolicited packets from the WAN (Internet) from entering the LAN Devices on the Internet cannot directly connect to devices on your LAN, Has to keep track of connections so that valid packets from the WAN can enter the LAN Devices on your LAN can connect to servers on the Internet The firewall allows the “return” packets to pass back into the LAN Usually allows for packet forwarding / virtual servers –Allows you to run servers on you LAN This allows devices on the WAN (Internet) to indirectly get to devices on your LAN if you explicitly configure it

DHCP Dynamic Host Configuration Protocol Automatically assigns an IP address (and other configuration information) to each of the internet device attached to the LAN segment Configuration Information includes: IP Number Subnet Mask Default Gateway DNS Servers

NAT Server Network Address Translation Allows the DHCP server to hand out “local” IP addresses instead of “real” IP numbers to the devices on the LAN network When a packet from one of the LAN devices needs to be routed to the Internet, the NAT rewrites the packet’s “from” address with the “real” address of the router before it sends it –The NAT keeps track of the connections / conversations so that it can rewrite the addresses correctly –The access router got the “real” IP address from the cable/DSL modem –Devices on the Internet can directly send to (and receive from) the “real” IP address, but not to the “local” IP address of the LAN devices When a packet comes from the internet to the “real” address, the NAT looks up the connection information and rewrites the packet’s “to” address to send it to the correct local device ---

IP Address 32 Bit Number –Expressed in dotted decimal notation: – Every computer on the Internet has to have an IP address –in order to send IP messages IP addresses can be “real” or “local”

“Real” IP Addresses First part of address is “network” part –Network part is assigned to your ISP x.x is assigned to UTM –Addresses are assigned in blocks to ISPs –Allows packet to be routed / forwarded across Internet to host You get “real” IP Addresses from your ISP –The ISP usually charges for extra IPs We are running out of “real” IPv4 addresses –No two Internet devices can share the same “real” IP address –Some ranges are reserved –Addresses are assigned in blocks

Local IP Addresses Certain ranges of IP addresses are “local” addresses –Example local IP range: x.x 10.x.x.x Local IP addresses are NOT route-able on the Internet –Their “network” part is not assigned to any ISP Local IP addresses are “free.” One of the main reasons we all have not had to switch to IPv6 is because of the use of local addresses –many devices can use the same “local” IP address as long as they are not on the same LAN

Sending an IP Message Locally Most PCs use a simple set of rules to send an Internet message If the IP address of the destination is in the same subnet: –Use the ARP protocol to find the Ethernet address of the host that is using the destination IP Simply broadcast a packet to all machines on the LAN asking “who has IP x.x.x.x” Wait for the answer –Send the IP packet in an Ethernet frame directly to the destination Ethernet address

Sending an IP Message Afar If the IP address of the destination is NOT in the same subnet: –Use the ARP protocol to find the Ethernet address of the “default gateway” ROUTER We learned our default gateway from DHCP Simply broadcast a packet to all machines on the LAN asking “who has IP x.x.x.x” Wait for the answer –Send the IP packet in an Ethernet frame directly to the router’s Ethernet address The router will forward the packet onward

Subnet Mask Device learns its subnet mask from DHCP The subnet mask allows it to determine if another IP address is local (on the same LAN) or not Tells the number of bits that are the network part of our IP address Example: – Says that the first two bytes of the IP are the network part – Says that the first three bytes are the network part

Packets are Contained in Frames Packets are encapsulated in frames A frame only travels across a single network segment A packet travels all the way from the IP source host to the IP destination host Switches look at the frame header –They look at the Ethernet addresses Routers look at the IP header and packet –They look at the IP addresses Ethernet TO/FromIP TO/From IP DATA Frame HeaderFrame Data: IP Packet