What does Router MTU mean? If you can't post WeChat images or some websites won't open no matter what, it's probably because the numbers aren't right

last year I helped my cousin investigate a strange incident: WeChat text replies instantly, but as soon as the picture is posted, it spins in circles, then turns into a red exclamation mark after a long time; The bank's official website and company OA just won't open, but Baidu and Bilibili are all normal. Testing over 900 Mbps at full speed, restarting the router, changing DNS, uninstalling and reinstalling WeChat, all of it was useless. After struggling until 10 p.m., I finally found a strange number in the router's WAN settings—MTU, which was set to 1500. Changed to 1492, and recovered on the spot.

first put MTU in plain language: one carriage has a weight limit 1500

MTU stands for Maximum Transmission Unit, which literally means maximum number of bytes a network packet can hold Ethernet's rule is 1500 bytes. You can think of it as the weight limit for a delivery truck: your data must be loaded first, and each package must not exceed 1500 jin (about 1500 jin). If it goes over, it must be split into two pieces for separate transport.

unpacking (called slicing in jargon) isn't a mistake in itself, but it's laborious: unpacking on the road, reassembling upon arrival, and starting over from scratch if you get a wrong package. So when it comes to web pages and TCP traffic, they put a 'Don't dismantle me' label when outsourcing—in jargon, it's called a DF logo. It means if you find any section of the journey can't fit it, notify the shipper in advance so they can switch to a new vehicle and reinstall it.

pitfalls are the real problems—some devices on the way are too lazy to notify you. bags that couldn't fit were thrown away without a word. the shipper is still waiting for the receipt, waiting for the delay, reshipping, then being thrown away, and overtime again. On your phone, what you see is: the page spins, and at the bottom, a message pops up saying "Inaccessible." This phenomenon is called a PMTU black hole in jargon, which is colloquially described as "a package thrown away halfway and you won't be told back." All the strange phenomena at my cousin's house that night were performances of this mechanism.

Why dial-up internet requires 1492: 8 seats are occupied in the carriage

was perfectly fine, why couldn't the 1500 fit it all? Because it uses broadband dial-up internet, which is PPPoE—the connection method for entering your ISP account and password. I previously wrote about this connection to web settings in the article on router automatic dial-up . PPPoE requires putting 8 bytes of "bills" (contract header expenses) in each bag, meaning a 1500-jin carriage is fixed to occupy 8 jin of seats, so you can actually hold 1492.

cousin's home originally used optical modem router modem, with the optical modem dialing for him, and the MTU matter was quietly handled behind the scenes. A while ago, I helped him switch the optical modem to bridge, and the router dialed up by itself (I noted the aftermath of the bridge switch in IPTV post), and the WAN port MTU on his old firmware router was set at 1500 by default—from that day on, all packets over 1492 were lost at the carrier's side.

by the way, some families often fall into this trap: switching the optical modem to bridge your own dialing number; The second most common player is those who have used software routers or flashed computers, and this number is most likely to be lost when migrating new firmware configurations; And the one who took over the second-hand machine—you don't know what the previous owner messed with, but my cousin's router was the one I fished out of Xianyu. If you've been living quietly in optical modem router mode, you'll basically never encounter it.

led to the strange phenomenon of "small packets being accepted, big packets lost": WeChat text is just a few hundred bytes, and the green light arrives instantly; A 3MB original image had to be cut into two thousand or so 1,500 yuan large packages, each discarded. Keep the comparison chart below for now; if your symptoms match, then read on:

three steps to measure the numbers you need to fill in at home

don't just memorize 1492 and force it in—measure your home numbers yourself, three steps, and just use Windows' built-in ping.

Step , connect your computer to your home network, open the command prompt, and type:

ping -f -l 1472 114.114.114.114

the accounting is calculated like this: 1472 is the loaded goods, plus the 28-byte header for 1500. -f means "No unpacking," and -l 1472 the specified weight. This trip was like sending a top-tier car to cover the entire route.

the second step, let's see the result. If it works, it means you can drive 1500 km from your home to the carrier, and the MTU is fine—don't touch it. If it doesn't work, you'll get a message: 'Need to split but DF is set.' Good, it means there really is a narrow bridge on the road—keep trying down:

ping -f -l 1464 114.114.114.114

1464 still not working, then 1452 and 1440, each bar lowered. My cousin's family reported 1472 errors and 1464 calls.

Step 3 , add 28 to the number of 'Tong'. 1464+28=1492, which is the MTU they should fill in. By the way, it's best to connect the router directly to the router with a network cable to eliminate wireless interference; Occasionally losing one or two packets doesn't count; it only counts when the clear reply "needs to split packets." Packet loss and fragmenting are two different issues. I wrote about packet loss troubleshooting in the article WiFi packet loss rate . Also, this job can't be done by phone; iOS and Android don't give 'No Unpacking' ping permissions. If you don't have a computer at home, borrowing a laptop from a neighbor takes ten minutes.

where do brands hide this option

the test is out, then go to the backend to fill it out. Each school's Tibetan method is different:

two reminders. First, TP's official standards are very strict: PPPoE connection must be 1492 or lower, no negotiation. Second, there's a blunt truth in Xiaomi's official FAQ—'If a normal website can be loaded, there's no need to change the MTU value.' If you run into trouble, they'll suggest you try 1412. After filling out the form and saving it, let the router disconnect and redial once. Changing the simple change without redialing will not work.

my home uses optical modem router mode and has never dialed numbers myself. The optical modem handles the entire process for you, so if you can't find this option in the backend, there's no need to bother with it.

changing it big or small comes at a cost; it's not an optimization knob

some people think this number is like gain—the bigger it gets, the faster. Twisting it from 1500 to 1800—that's a dead end for home use. Your switch, optical modem, and carrier equipment all operate according to the 1500 yuan rule. If any device on the road doesn't recognize it, the data will be cut off immediately, and it's the hardest-to-verify case of "some connecting, some not." One of the detours my cousin took that night was actually from here.

minor modifications are safe, but the trade-off is that each package contains less cargo and large file transfers are a bit slower, but the result is that every bag can be processed. The scenario where you really need to proactively downsize is VPN: after connecting to the company VPN, the images in the OA can't be displayed—the same issue—the VPN tunnel is wrapped in an encrypted layer on the outside of the package, and the cabin is narrower, which is common between 1400 and 1470. In this case, you can modify it on your computer without touching the router:

netsh interface IPv4 set subinterface "Ethernet" mtu=1400 store=persistent

After the modification, disable the network card before reactivating it. Let's first look at the MTU of each local network card, using netsh interface IPv4 show subinterfaces, with one line each for WiFi and Ethernet.

go deeper: when you shake hands with TCP, both ends negotiate an MSS (1492 corresponds to 1452, minus the 40-byte TCP connector). If older firmware changes to MTU, you still have to manually change MSS, while newer firmware automatically syncs. OpenWrt players need to worry about this; home routers just need to know about this. In the router's backend, there is a parameter like this for "don't need to worry about it normally, only becomes aware when something happens," and DHCP address pool counts one; I've written them all before.

take it personally—check for every symptom

'Website won't load' is a cure-all—triage before you act, don't change parameters right away

to wrap up, here's the order: first confirm that all the packets are working (text, search, and web pages are normal). → starting from 1472→ if it's not working, find the highest number that can pass→ add 28, enter it into the WAN port→ save and redial, then open the website that previously wouldn't open. If it still has issues, lower it to 1460 and try again. The problem remains, which means the numbers are correct but the fault isn't theirs. Check the wiring and light decay later. How to test speed without getting tricked by the stance? I wrote about it in my article about broadband speed , for matching purposes.