Question: Both the FastAPI backend and the Next.js frontend are running on localost. On the same computer, the frontend makes API calls using fetch without any issues. However, on a different computer on the same network, e.g., on 192.168.x.x, the frontend runs, but its API calls are no longer working. I have tried using a proxy as next.js but that still does not work. Frontend:  export default function People({setPerson}:PeopleProps) { const fetcher = async (url:string) => await axios.get(url).then((res) => res.data); const { data, error, isLoading } = useSWR(`${process.env.NEXT_PUBLIC_API}/people`, fetcher); if (error) return <div>"Failed to load..."</div>; return ( <> {isLoading? "Loading..." :data.map((person: Person) => <div key={person.id}> {person.name} </div>)} </> ) }  The Next.js app loads the env.local file at startup, which contains: NEXT_PUBLIC_API=http://locahost:20002 Backend: rom typing import List from fastapi import APIRouter, Depends from ..utils.db import get_session as db from sqlmodel import Session, select from ..schemas.person import Person, PersonRead router = APIRouter() @router.get("/people", response_model = List[PersonRead]) async def get_people(sess: Session = Depends(db)): res = sess.exec(select(Person)).all() return res  The frontend runs with: npm run dev, and outputs ready - started server on 0.0.0.0:3000, url: http://localhost:3000  The backend runs with: uvicorn hogar_api.main:app --port=20002 --host=0.0.0.0 --reload, and outputs: INFO: Uvicorn running on http://0.0.0.0:20002 (Press CTRL+C to quit)  When I open the browser on http://localhost:3000 on the same machine the list of Person is displayed on the screen. When I open the browser on http://192.168.x.x:3000 on another machine on the same network, I get the "Failed to Load..." message. When I open the FastAPI swagger docs on either machine, the documentation is displayed correctly and all the endpoints work as expected. CORS look like this: from fastapi import FastAPI from fastapi.middleware.cors import CORSMiddleware app = FastAPI() origins = [ "http://localhost:3000", ] app.add_middleware( CORSMiddleware, allow_origins=origins, allow_credentials=True, allow_methods=["*"], allow_headers=["*"], ) 

Answer: Setting the host flag to 0.0.0.0 To access a FastAPI backend from a different machine/IP (than the local machine that is running the server) on the same network, you would need to make sure that the host flag is set to 0.0.0.0. The IP address 0.0.0.0 means all IPv4 addresses on the local machine. If a host has two IP addresses, e.g., 192.168.10.2 and 10.1.2.5, and the server running on the host listens on 0.0.0.0, it will be reachable at both of those IPs. For example, through command line interface: uvicorn main:app --host 0.0.0.0 --port 8000  or, programmatically: if __name__ == '__main__': uvicorn.run(app, host='0.0.0.0', port=8000)  Note that RFC 1122 prohibits 0.0.0.0 as a destination address in IPv4 and only allows it as a source address, meaning that you can't type, for instance, http://0.0.0.0:8000 in the address bar of a web browser and expect it to work. You should instead use one of the IPv4 addresses of the local machine, e.g., http://192.168.10.2:8000 (or, if you are testing the API on the same local machine running the server, you could use http://127.0.0.1:8000 or http://localhost:8000). Adjusting Firewall Settings You may also need to adjust your Firewall to allow external access to the port you specified, by creating an inbound firewall rule for Python. On Windows, this is usually created automatically, when allowing a program—Python, in this case—to communicate through Windows Firewall, and by default this allows traffic on Any port (for both TCP and UDP connections). Adjusting CORS Settings Additionally, if your frontend is listening on a separate IP address and/or port number from the backend, please make sure to have CORS enabled and properly configured, as desribed in this answer and this answer. For example: origins = ['http://localhost:3000','http://192.168.178.23:3000'] app.add_middleware( CORSMiddleware, allow_origins=origins, allow_credentials=True, allow_methods=["*"], allow_headers=["*"], )  Making HTTP requests in JavaScript Finally, please take a look at this answer and this answer regarding using the proper origin/URL when issuing a JavaScript fetch request from the frontend. In short, in your JavaScript asynchronous request, you should use the same domain name you typed in the address bar of your web browser (but with the port number your backend server is listening on). If, for example, both the backend and the frontend server are listening on the same IP address and port number, e.g., 192.168.178.23:8000—that would be the case when using Jinja2Templates for instance—you could access the frontend by typing in the address bar of the browser the URL leading to the frontend page, e.g., http://192.168.178.23:8000/, and the fetch request should look like this: fetch('http://192.168.178.23:8000/people', {...  For convenience, in the above case—i.e., when both the backend and the frontend are running on the same machine and listening on the same port—you could use relative paths, as suggested in a linked answer above. Note that if you are testing your application locally on the same machine and not from a different machine on LAN, and instead using 127.0.0.1 or localhost to access the frontend/backend, those two are different domains/origins. Thus, if you typed http://127.0.0.1:8000/ in the address bar of the browser to access the frontend page, you shouldn't make fetch requests using, for instance, fetch('http://localhost:8000/people', as you would get a CORS error (e.g., Access to fetch at [...] from origin [...] has been blocked by CORS policy...). You should instead use fetch('http://127.0.0.1:8000/people', and vice versa. Otherwise, if the frontend's origin differs from the backend's (see this answer for more details on origin), it should then be added to the list of origins in the CORS settings of the backend (see example above).