Network Boundaries and Troubleshooting

Network Boundaries and Troubleshooting

• Routers act as gateways, enabling communication between different networks.
• Each router interface connects to a separate network.
• The IPv4 address assigned to an interface identifies its connected local network.
• Host PCs typically set the router as their default gateway.

Routers as DHCP Servers

• DHCP (Dynamic Host Configuration Protocol) is a network server that automatically assigns IP addresses, default gateways, and other network parameters to client devices.
• Wireless routers configured as DHCP servers automatically provide the correct interface IPv4 address to hosts as their default gateway.
• The default IPv4 address on a local wireless router is often the first host address on that network.
• Internet Service Providers (ISPs) use DHCP servers to provide IPv4 addresses to the internet side of routers, often termed the external or outside network.
• When connected to an ISP, a wireless router acts as a DHCP client to receive an external network IPv4 address for its internet interface, enabling internet access.

Importance of Routing

• Routers connect multiple Layer 3 (logical) IP networks.
• Switches make forwarding decisions based on Layer 2 (physical) MAC addresses.

Routing Table Entries

• Routing tables store information about networks and optimal paths to reach them.
• Entries can be dynamically updated from other routers or manually configured.
• If a router cannot determine a route, it drops the message.
• A default route ensures packets aren't dropped if the destination network isn't listed.

Assessment (Host Configuration)

• Default gateway configuration allows local network computers to communicate with devices on other networks.

Assessment (Devices with Specific Gateways)

• Default gateways for different hosts (H1, H2, H3) are listed as 192.168.1.1, 10.0.0.1, and 172.16.0.50 respectively.

Network Address Translation (NAT)

• NAT converts private addresses to public internet-routable addresses.
• Private addresses are translated to public, and vice versa, for incoming packets.
• NAT improves flexibility and security but breaks end-to-end IP traceability and potentially causes delays.

NAT Types

• Static NAT: One-to-one mapping of private to public IP addresses.
• Dynamic NAT: Uses a pool of public IPs dynamically assigned to private IPs.
• Port Address Translation (PAT): Maps multiple private hosts to a single public IP address using unique port numbers.

Network Address Translation - Terms

• Inside Local: Private IP address within a local network.
• Inside Global: Public IP representing a local device on the internet.
• Outside Local: Public IP viewed from a local perspective.
• Outside Global: Public IP of an external device on the internet.

How NAT Works

• Hosts send packets with private IP addresses.
• The router translates the private IP to a public IP and logs it in the NAT table.
• The router forwards the packet with the public IP.
• The external host responds to the public IP.
• The router translates the public IP back to the private IP and delivers the response.

How NAT Works (PAT)

• Multiple private hosts share a single public IP.
• Assigned unique ports help distinguish each host's communication.
• Router translates the private IP and port numbers to public IP and port numbers for outgoing traffic.
• The router reverses the translation for incoming traffic, routing to the appropriate private host.

IP Addresses (Private vs Public)

• 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 are private IP address ranges.

Advantages of NAT

• NAT allows many private IP addresses to share one public IP address, conserving public IP addresses.

IP Addressing Basics

• IP addresses are unique identifiers for devices on a network.
• Subnet masks determine how the IP address is divided into network and host portions.
• The default gateway routes traffic outside the local subnet.
• Common IP addressing issues include overlapping IP addresses, incorrect subnet masks, and mismatched gateways.

Command-Line Tools

• Ping: Uses ICMP to test host IP stack initialization.
• Traceroute: Measures transit delays of packets across a network.
• ARP (Address Resolution Protocol): Resolves IP addresses to MAC addresses, bridging Layer 3 and Layer 2 communication on a local network.
• ipconfig /all: Displays PC network configuration.

IP Troubleshooting

• Steps for troubleshooting IP issues:
  1. Verify IP configuration.
  2. Check subnet mask and gateway.
  3. Analyze network topology.
  4. Test communication.

IP Troubleshooting Analysis

• Valid hosts and broadcast addresses are checked for proper configuration in various network segments.
• Hosts should not use broadcast addresses as IP addresses.

Checkpoint Questions (Various)

• Answers to various checkpoint questions are provided, focusing on device suitability for NAT implementation, desirable IP configuration settings for subnet communication, and the technology to handle numerous users needing internet access from a limited public IP address range.