Fundamental networking concepts
- 2. www.innervoice.in Agenda • Concepts – Subnet Mask – Default Gateway – DNS Server • Context – a typical home network • Assumptions – Familiarity with DHCP fundamentals – Familiarity with OSI stack for networking. 2
- 3. www.innervoice.in A typical home network 3 ISP’s Network Internet google.com Web ServerDNS Server Gateway DSL or Cable RJ-45PortsforLAN DHCP Server
- 4. www.innervoice.in Understanding Network Configuration 4 • As you can see the common information is – Subnet Mask – Default Gateway (Windows), Router (IPad) and Default Route (Ubuntu) – DNS Servers (Windows), DNS (IPad) and Primary DNS (Ubuntu)
- 5. www.innervoice.in OSI Stack View of our Network 5 ISP’s Network Internet google.com Web ServerDNS Server PHYSICAL DATA LINK NETWORK TRANSPORT APPLICATION PRESENTATION SESSION Home Computer PHYSICAL DATA LINK NETWORK TRANSPORT Home Router Wired/Wireless connection PHYSICAL DATA LINK NETWORK TRANSPORT APPLICATION PRESENTATION SESSION Google Web Server Internet Internet DNS Server PHYSICAL DATA LINK NETWORK TRANSPORT APPLICATION PRESENTATION SESSION Gateway RJ-45PortsforLAN DHCP Server DSL or Cable
- 6. www.innervoice.in The first step… • What happens when you type www.google.com on you browser? • Application Layer: Browser will create a HTTP get request • Network Layer: Needs to create an Network layer packet (IP Packet) to be sent to www.google.com. • But Network Layer needs IP Address not URL. • This is where DNS Server comes into picture 6
- 7. www.innervoice.in 7 PHYSICAL DATA LINK NETWORK TRANSPORT APPLICATION PRESENTATION SESSION Home Computer PHYSICAL DATA LINK NETWORK TRANSPORT Home Router ISP’s Network Internet The browser sends a request to local DNS Client application to convert the URL of google.com to an IP address. DNS Server PHYSICAL DATA LINK NETWORK TRANSPORT APPLICATION PRESENTATION SESSION Communicating with DNS Server An IP Packet is constructed containing: • Source IP: Home Computer • Destination IP: DNS Server The DNS Server IP is part of network configuration (DHCP response) The IP Packet or the L3 Packet is then wrapped into an L2 Frame and sent on the wire to the DNS Server. The DNS Server then responds back with the IP Address of google.com
- 8. www.innervoice.in L3 to L2 Conversion • Layer 3 or L3 uses IP Address of a device • Layer 2 or L2 uses Physical Address of a device – For Ethernet the physical address is the MAC address of the Network Port. • An L2 frame = Entire L3 packet + Some L2 information. The most important are: – source MAC address – destination MAC address • In our example, – the source MAC address = Home computer MAC address – But how can we find the destination MAC address? 8
- 9. www.innervoice.in Destination MAC Address Flow 9 Apply ‘subnet mask’ on destination IP Address (DNS Server) Is the resulting IP in the same sub- network as the source IP Address? Assume Destination IP = Default Gateway IP Yes No Use ARP Tables to lookup MAC address for the Destination IP Construct L2 Frame with Destination MAC In our example, the IP address of the DNS Server is outside the home computer's sub-network. Hence Destination MAC address = MAC address of the ‘Default Gateway’.
- 10. www.innervoice.in The role of Default Gateway (Router) • Default gateway is used for 'routing' network packets between one more LANs or between a LAN and a WAN. 10 Examine the incoming L2 Frame Does the destination MAC address contain my MAC address? Drop the packet Yes No Extract the L3 Packet from the L2 Frame Forward/Route the L3 Packet based on Destination IP Address Note: while forwarding L3 Packets: Source IP = Home Computer IP Destination IP = DNS Server IP
- 11. www.innervoice.in What happens to incoming L3 packet? 11 Examine the incoming L3 Packet (destined for Home Computer) Can I reach the destination IP on my LAN? Drop the packet Yes No Convert an L3 Packet to an L2 Frame Forward the L2 Frame based on Destination MAC Address Note: while forwarding L2 Frames: Source IP = DNS Server IP Destination IP = Home Computer IP Source MAC = Default Gateway MAC Destination MAC = Home Computer MAC
- 12. www.innervoice.in 12 PHYSICAL DATA LINK NETWORK TRANSPORT APPLICATION PRESENTATION SESSION Home Computer PHYSICAL DATA LINK NETWORK TRANSPORT Home Router ISP’s Network Internet The browser sends a request to local DNS Client application to convert the URL of google.com to an IP address. DNS Server PHYSICAL DATA LINK NETWORK TRANSPORT APPLICATION PRESENTATION SESSION Communicating with DNS Server An IP Packet is constructed containing: • Source IP: Local Computer • Destination IP: DNS Server The DNS Server IP is part of network configuration (DHCP response) After IP to MAC lookup the L2 packet will contain the following address information: Source IP = Home Computer IP Destination IP = DNS server IP Source MAC = Home Computer MAC Destination MAC = Default Gateway MAC The L2 Packet received by the Home Router since its MAC address was in the destination address. The L3 packet inside this L2 frame tells the ‘Gateway’ or ‘Router’ to forward the packet to the DNS Server. The DNS Server then responds back with the IP Address of google.com
- 13. www.innervoice.in And it starts all over again • At this stage the home computer has the IP address of www.google.com • Next the L3 frame meant for www.google.com is constructed. • Once again the whole process of L3 to L2 frame conversion and IP-to-MAC lookup will take place. • And just like DNS server responded to a DNS request, the Google Web Server will respond to a HTTP Get request. 13
- 14. www.innervoice.in Summary • DNS Server is used to convert a URL to an IP address • Subnet Mask is used for determining whether IP to MAC lookup should be done or not. • Default Gateway is the entity that routes packets from LAN to WAN (ISP/Internet) and vice-versa 14