What's default gateway?

What's default gateway?

A default gateway is a network device that acts as an entry or exit point for traffic to and from a network. In other words, it is the router or layer 3 switch that connects a local network to the internet or another network.

When a device such as a computer or server needs to communicate with a device on a different network, it sends the traffic to its default gateway. The default gateway then routes the traffic to its destination network. Similarly, when a device on a different network needs to communicate with a device on the local network, it sends the traffic to the default gateway, which then routes the traffic to its destination on the local network.

The default gateway is usually set to the IP address of the router or layer 3 switch that connects the local network to the internet or another network. Devices on the local network are configured with the IP address of the default gateway as part of their network configuration, so they know where to send traffic that is not destined for the local network.

show route table for ipv4

route print -4

To show the route table for IPv4 on a device, you can use the command prompt or terminal of the device to run the appropriate command. The specific command may vary depending on the operating system and device, but here are a few examples:

On Windows:

Open the Command Prompt and run the command "route print". This will display the routing table for IPv4 on the device.

On Linux:

Open the Terminal and run the command "ip route show". This will display the routing table for IPv4 on the device.

Alternatively, you can also run the command "route -n" to display the routing table for IPv4 in a more concise format.

On macOS:

Open the Terminal and run the command "netstat -rn". This will display the routing table for IPv4 on the device.

In general, the route table for IPv4 contains information about the network destinations that the device can reach, the gateway or next hop that the device should use to reach each destination, and other related information such as the interface used to reach each destination. The route table is used by the device to determine the best path for routing traffic to its destination.

What's ICMP?

ICMP stands for Internet Control Message Protocol. It is a network protocol used for communication between network devices, primarily for diagnostic and troubleshooting purposes.

ICMP is used to send error messages and operational information about network conditions. For example, if a packet is lost or a network device is not reachable, an ICMP message can be sent to the source device to notify it of the problem. ICMP is also used by some network tools and utilities, such as ping and traceroute, to test connectivity and diagnose network issues.

Some common ICMP messages include:

Echo Request and Echo Reply: Used by the ping utility to test connectivity by sending a request to a device and waiting for a reply.

Destination Unreachable: Sent by a router or firewall to indicate that a network or host is unreachable.

Time Exceeded: Sent by a router to indicate that a packet has been discarded because it exceeded the maximum time allowed for it to traverse the network.

ICMP is an integral part of the Internet Protocol (IP) suite and is used by all IP-based networks. It operates at the network layer of the OSI model and does not require any additional protocols or services to function.

The network is good, but ping did not work

If the network is good, but you are unable to ping a device, there are several possible reasons for this:

The device may have ICMP disabled: Ping uses ICMP packets to test connectivity, so if ICMP is disabled on the device you are trying to ping, you will not receive a response. Check the device's firewall settings and network configuration to ensure that ICMP is enabled.

The device may not be configured to respond to ping requests: Some devices may be configured not to respond to ping requests as a security measure. This is common for servers and other critical network devices. Check the device's configuration to see if it is configured to respond to ping requests.

There may be a network issue: Even if the network appears to be good, there may be issues such as packet loss or high latency that can affect connectivity. Use other tools such as traceroute or pathping to diagnose potential network issues.

Incorrect IP address: Ensure that you are pinging the correct IP address for the device you are trying to reach. Double-check the IP address and subnet mask configuration on both the source and destination devices.

Firewall blocking ping requests: Check if the firewall on the source device is blocking outgoing ping requests or if the firewall on the destination device is blocking incoming ping requests.

By troubleshooting the above issues, you can identify the cause of the ping failure and take appropriate steps to resolve it.

How tracert work with ttl?

Tracert, also known as traceroute, is a network diagnostic tool used to trace the path that an IP packet takes from a source device to a destination device over a network. Tracert works by sending packets with incrementally increasing Time to Live (TTL) values, and observing the response from each intermediate network device.

When a packet is sent from a source device, it has a TTL value that is set by the operating system. The TTL value indicates how many hops (network devices) the packet can travel before it is discarded. Each time the packet passes through a network device, the TTL value is decremented by one. If the TTL value reaches zero, the packet is discarded and an ICMP Time Exceeded message is sent back to the source device.

Tracert works by sending a series of packets with incrementally increasing TTL values, starting with a TTL value of one. The first network device that receives the packet will decrement the TTL value by one, and if the new TTL value is greater than zero, the packet is forwarded to the next device in the path. This process is repeated until the packet reaches its destination or the TTL value reaches zero, at which point an ICMP Time Exceeded message is sent back to the source device.

By observing the TTL values in the ICMP Time Exceeded messages, tracert can determine the number of hops between the source and destination devices, and can also identify any network devices that may be causing issues or delays. Tracert can also display additional information such as the IP addresses and hostnames of each intermediate device, as well as the response time for each hop.

So both ping and tracert based on ICMP? If the domain diabled ICMP, then we can't ping and tracert

Yes, both ping and tracert use ICMP packets to perform their functions. If a domain or device has ICMP disabled, you will not be able to ping or traceroute to that domain or device. This is because ping and traceroute rely on receiving ICMP packets in order to determine the status of the network path between the source and destination devices.

If you are unable to ping or traceroute to a domain or device, it may be due to ICMP being disabled, or it may be due to other network issues. In some cases, firewalls or network security policies may block ICMP traffic, which can also prevent ping and traceroute from functioning properly. If you suspect that ICMP is being blocked or disabled, you should check the device or network configuration to ensure that ICMP traffic is allowed.

有些公司为了安全问题和流量问题，就直接禁用ICMP

What's the protocol used for telnet?

The Telnet protocol uses the Transmission Control Protocol (TCP) as its transport protocol. TCP is a reliable, connection-oriented protocol that provides reliable data delivery over IP networks. Telnet uses TCP port 23 as its default port for communication between the Telnet client and server.

When a Telnet client connects to a Telnet server, a TCP connection is established between the two devices. The Telnet client then sends commands and data to the server using the Telnet protocol, which encapsulates the data within TCP segments. The Telnet server receives the data, processes it, and sends a response back to the client using the Telnet protocol over the same TCP connection.

Because Telnet uses TCP as its transport protocol, it is able to provide reliable, error-checked communication between the client and server. However, Telnet is considered to be an insecure protocol, as it sends all data, including usernames and passwords, in clear text over the network. As a result, Telnet is often replaced by more secure protocols such as SSH (Secure Shell).

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