Does a TCP packet change source ip when it goes through your router?

[saminator910]

I was wondering, every time I send or receive a TCP packet with an IP header, my local ip, ie. 192.168.1.x is displayed as the source or destination, respectively. Does this get changed as soon as the packet passes through my router, so the source/destination is listed as my external IP address?

 

[Psinter]

Small correction: Your external IP address is on the body, never on the header. IPs on the header are to communicate between local applications. Whether that header IP gets dropped in the computer or on the router, I don't know (which is basically what you asked). :( Maybe someone could provide some more insight.

 

[voko]

Your "router" is not just a "router" in the original meaning of the word. It is also a "network address translation" (NAT) unit.

Pure routers do not change IP addresses. They just pass the IP packet to whatever physical network is likely to transport it to its destination.

NATs do change addresses. That is why they exist. They map multiple "internal network addresses" to a single (sometimes also multiple) "external network address".

 

[rcgldr]

This depends on how your modem router function is configured. Typically the devices connected to a modem get an internal local IP and the router funcion of the modem converts these to an external IP with port numbers that vary depending on the local IP's so that it knows which external packets belong to which internal local IP. The alternative is the router function of the modem assigns an external IP to the devices connected to the modem, essentially converting the in home lan into an extension of the internet. From what I've read, all cable modems can do this for at least 16 external ips, but the internet service providers limit this, for example my ISP only allows 3 external ips for a home account. The home dsl modems that do this typically provide 8 static external ip's but use 3 of these, allowing up to 5 connected devices to have independent external ip's. Note that one or more of those connected devices could be standard routers, which would be assigned external ip's and in turn assign local ip's to the devices connnected to the router.

Getting back to the original question, if the modem assigns external ips to devices connected to the modem, then the modem acts like a bridge and does not change packet ip information.

Side note, in the case of a cable modem, if it's disconnected from the coax cable (or the cable service isn't working), it then assigns local ip's, and the home network will behave as a lan. I don't know if a dsl modem does the same thing if disconnected from the phone line.

 

[LudusRex]

Yes, a TCP packet will change its source IP when it goes through a router. This is because a router is responsible for forwarding packets between different networks, and each network has its own unique IP address range. When a packet enters a router, it is assigned a new source IP address that corresponds to the network it is being forwarded to. This new source IP address will be the external IP address of the router.

The process of changing the source IP address is known as Network Address Translation (NAT). NAT allows multiple devices on a private network, such as a home network, to share a single public IP address. This allows for more efficient use of limited IP addresses and helps to secure the private network by hiding its internal IP addresses from external networks.

In addition to changing the source IP address, a router may also modify other fields in the IP header, such as the destination IP address, in order to properly route the packet to its intended destination. This process is known as routing.

It is important to note that not all routers perform NAT. Some routers, such as those used in large enterprise networks, may simply forward packets without changing any of the IP header information. In these cases, the source IP address of the packet will remain the same as it passes through the router.

In conclusion, a TCP packet will typically have its source IP address changed when it goes through a router, as part of the NAT process. This allows for efficient routing of packets between different networks and helps to secure private networks. However, there are exceptions to this, as some routers may simply forward packets without changing any of the IP header information.